Although invisible, black holes betray their presence.  It's the same with dark matter - perhaps the missing 90% of the Universe.
 

Outline:

Black holes are invisible.  So too is dark matter - hidden mass that constitutes perhaps 90 per cent of the Universe.  Invisible they may be, but the effects of black holes and dark matter can be observed.  The outer parts of galaxies spin faster than their visible mass justifies, indicating the presence of hidden mass.  Gravitational interaction within star clusters suggests they contain up to 100 times more matter than can be seen.  Yet the nature of dark matter is a mystery.

How do black holes betray their existence?  Look for a star continuously losing mass and orbiting apparently empty space.  A black hole often forms in the aftermath of a supernova - where the exploding star has a core with a mass at least three times that of our Sun.  Other evidence of an invisible presence is the distortion of space and light at the rim of a black hole.  Anything and everything caught by a black hole is forever trapped.  Even light cannot escape.  Approach too close and the result is "spaghettification".

The bending of light may also disclose the presence of dark matter.  Light travelling to Earth can be distorted by the gravity of a massive intevening object.  So-called "gravitational lensing" and the degree of light-bending can reveal the amount of dark matter in the Universe.

At the centre of galaxies, supermassive black holes are believed to lurk.  They may have swallowed the equivalent mass of millions of Suns.  Thirty-thousand could lie within a supermassive black hole at the centre of our own Milky Way.   So what makes a supermassive black hole?  They may be triggered by galactic mergers or collisions.  The most powerful black holes are associated with fiercely bright quasars.  Quasars are fuelled by the hungriest black holes - consuming the equivalent of 600 Earths per hour.

Albert Einstein accurately predicted the effects of supermassive objects in space.  A final thought - could a black hole be a wormhole to another part of the cosmos or even to another Universe?
 
 

Sub-chapters:

The Invisible Universe

*  Black holes are invisible, their gravity so powerful that not even light can escape.

*  Nine-tenths of the Universe is thought to be invisible.  This "dark matter" is undetectable at any wavelength from radio waves to gamma-rays.

*  Dark matter may be largely responsible for the development and structure of the Universe.

*  The gravity of unseen objects betrays their presence.

*  Outer parts of galaxies spin faster than their visible mass justifies.  Dark matter, up to ten times the visible mass, accelerates the spin of these galaxies.
 

What Is Dark Matter?

*  Dark matter is not the visible black dust that peppers galaxies.  Often, clouds of such dust are where stars are born.  Invisible dark matter pulls on these stars.

*  Interactions between galaxies in clusters suggest up to 100 times more matter than can be seen.  Yet the nature of dark matter remains a mystery.  Could it be dead stars, failed stars, black holes or exotic sub-atomic particles?

*  When superstars die in supernovae, their cores collapse to form black holes - but only if the cores have at least three times the mass of the Sun.  A telltale sign of a black hole is a star continuously losing mass and orbiting "empty space".

*  A black hole can also be detected by the the distortion of light and space around its rim.

*  Material caught in the intense gravitational pull of a stellar mass black holes is spaghettified.
 

Bending Light

*  To become a black hole, Earth would have to be compressed into a ball just eight millimetres across.

*  Light can be bent by gravity - and it is one way of detecting dark matter.  If a massive object, such as a galaxy, is placed in the path of a beam of light, the light is distorted as though passing through a lens.  The galaxy acts as a "crazy lens", splitting the bent light into arcs and bright knots.

*  The degree of light bending enables scientists to "weigh" the intervening galaxy, revealing the weight of both the visible matter and the dark matter.
 

Galactic Black Holes

*  A swirling ring of gas at the heart of the Milky Way may be evidence of a massive black hole - with a mass 30,000 times that of our Sun.

*  Galactic mergers and collisions - where the galaxy cores unite - produce supermassive black holes.  The strange galaxy Centaurus A is an example.  At its centre is a black hole with a thousand times the mass of the black hole at the centre of the Milky Way.
 

Supermassive Black Holes

*  Supermassive black holes are surrounded by an accretion disk, matter swirling inwards due to intense gravitational pull.  Some matter jets away at right angles to the disk.  Such a supermassive hole consumes the equivalent of four Earths an hour.

*  The supermassive black hole at the core of the galaxy Virgo A has the mass of two-and-a-half billion Suns.  It fires a jet of material across space.
 

The Ultimate Distorter

*  Fiercely bright quasars play host to the most powerful black holes.  Quasars are fuelled by supermassive black holes that consume the equivalent of 600 Earths an hour.

*  Einstein's General Theory of Relativity - explaining how massive objects can bend light and distort space.

*  What's inside a black hole?  Could it be a wormhole to another part of the cosmos - or even to another Universe?
 
 

Background:
 

Black Holes

Black holes are regions of space where the gravitational field is so strong that photons - particles of light travelling at 300,000 kilometres per second - cannot escape.  Stellar mass black holes are formed when massive stars of greater than eight solar masses explode in a supernova.  Supermassive black holes are much larger.  They are formed during galactic mergers.

The first conclusive evidence for a supermassive black hole came in 1994.  It was discovered at the centre of the giant elliptical galaxy M87 by scientists using the Hubble Space Telescope (HST).  Their evidence came from the effect of the black hole on its surroundings.

The speed of objects surrounding a black hole indicate the amount of mass within a certain radius.  HST measured the speed of rotation of the accretion disk at the heart of M87.  It indicated a mass of 2,500 million solar masses concentrated within an area so small that, according to basic physics, light could not escape - the criterion for a black hole.

From the moment of this discovery, supermassive black holes were no longer theoretical concepts.  As well as lying at the hearts of many giant elliptical galaxies, radio galaxies, Seyfert galaxies and quasars, supermassive black holes are now believed to exist at the centre of many spiral galaxies - including our own Milky Way.  Scientists are trying to find the gravitational wave signatures of black holes as predicted by Einstein's General Theory of Relativity.

Back in 1783, the English cleric John Michell suggested that a "dark star" could be the product of a gravitational force so strong that light could not escape from it.  A hundred years earlier, Ole Romer had accurately measured the speed of light.  This enabled Michell to work out that our Sun - without changing its mass - would have to shrink to a body with a radius of just three kilometres if it were to prevent the escape of light.

Michell postulated that if such objects existed, light could not escape from them and they would be dark.  Little more than a decade later, the French mathematician Pierre Simon Laplace reached the same conclusion.  He published his ideas in1796 with the remark that, although these bodies might be invisible, we could infer their presence by the "revolvement of other luminiferous bodies around them".  So black holes would most easily be found if they occurred in binary star systems.

Not until 1916 did Albert Einstein's General Theory of Relativity show that gravity is related to the curvature of space and that a black hole is a region of space where curvature becomes so extreme that a hole forms.  Karl Schwarzschild analysed the implications of Einstein's theory.  He worked out a formula for calculating the radius of a black hole of any particular mass - in other words, the radius of the so called "event horizon", the point of no return beyond which light cannot escape.  This radius is now called the Schwarzschild Radius.
 

Is the Universe Open or Closed?

The amount of dark matter in the cosmos is crucial to answering the question of whether the Universe is "open" or "closed".  If it is open, then the total volume of space is infinite and the Universe will expand forever.   If it is closed, the Universe contains a finite amount of space and will eventually collapse back on itself - ending in a Big Crunch as the matter comes together.

It has been suggested that the Big Crunch could be followed by another Big Bang, giving rise to the birth of a new Universe.   If so, the Universe might oscillate between Big Bang and Big Crunch and could go on forever.   If the Universe is neither open nor closed it is said to be "flat".  Watch this space.
 

Dark Matter

It is possible that 99 percent of matter is hidden.  Cosmologists have pondered the form this dark matter might take.  Suggestions have included old dead stars, very low mass stars called brown dwarfs, rocky bodies the size of planets or asteroids, black holes or "exotic" particles.  Recently, in 2000, the very existence of dark matter has been seriously questioned.  Again, watch this space.
 

Gravitational Lenses:  Nature's Giant Telescopes

Nature has lent astronomers a hand with the discovery of "gravitational lenses".  These naturally occuring "telescopes" can be used to study quasars and the faint light of remote galaxies that would otherwise be beyond the reach of even the most powerful telescope.   As more gravitational lenses are identified, they promise to be a powerful tool in probing the early Universe.

If a faint, distant galaxy or quasar lies behind a nearer cluster of galaxies - and provided the cluster is sufficiently massive - then light rays from the distant object will be bent by the strong gravitational pull of the cluster.  The overall effect is that the cluster of galaxies acts like a gigantic glass lens - magnifying and distorting the image of the faint galaxy or quasar behind it and producing arcs or multiple images.

Gravitational lenses are already used to investigate the first era of star formation in high redshift galaxies, the mass of galaxy clusters, the existence of dark matter, the size and structure of distant quasars, as well as the size scale of the Universe.
 
 

Links for Further Information:

Black holes and beyond.  An interesting page with a brief history of the subject, reasons for studying black holes, a thorough account of black hole formation - and images.
http://www.ncsa.uiuc.edu/Cyberia/NemRel/BlackHoles.html

Good page on black holes - introductory information, images with accompanying text, clear diagrams to show the effects and behaviour of black holes.
http://www.damtp.ca.ac.uk/user/gr/public/bh_critical.html

Dark matter in the Universe.  An educational introduction to dark matter - its effects on galaxy clusters, gravitational lensing and images.
http://www.zebu.uoregon.edu/1996/ph123/19.html

An interesting picture on the appearance of the Universe with dark matter, plus accompanying text.
http://www.ucolick.org/~deep/overview/darkmatt.html

An excellent page on the theory of relativity.  Includes links relating to Einstein, gravitational lensing, pulsars and other relativity links on the web.
http://csep10.phys.utk.edu/astr162/lect/cosmology/gravity.html

Radio image of the region around the black hole at the centre of the Milky Way.
http://dnausers.d-n-a.net/dnetGojg/Black/mw.html
 
 
 

Questions and Activities for the Curious:

1.  Summarise the evidence supporting the idea that galaxies and galaxy clusters contain a great deal of dark matter.

2.  What is dark matter?  Give examples of what it might be.

3.  Describe the events leading up to the formation of a stellar mass black hole in a supernova explosion.

4.  What would happen to someone falling into a stellar mass black hole - and why?

5.  What does the term "escape velocity" mean?  Use it to explain why black holes are said to be "black".

6.  Explain the principle of gravitational lensing and give some examples.

7.  Why do astronomers believe that a supermassive black hole inhabits the centre of our Milky Way galaxy?

8.  What is meant by an "open" or "closed" Universe?  Explain why dark matter is important in this regard.