What is Dark Matter ?

Dark matter exerts ‘gravitational force’, meaning that it draws other matter towards it. And there’s so much dark matter that its gravitational force is enough to hold entire galaxies – like our own Milky Way – together. That’s why dark matter is often likened to a giant spider’s web, meshing galaxies in place.

The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction

We can also see the effects of dark matter simply by looking up at the sky. When astronomers observe distant galaxies, they often appear stretched and oddly shaped.

"empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands.

"empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands.

"empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands.

This effect is known as ‘gravitational lensing, and it’s caused by dark matter’s gravitational force. This force is so huge that it physically bends the light around galaxies, distorting their appearance.

As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster

By studying ‘relic radiation’ left over from the Big Bang, scientists can identify areas in space where more radiation exists – and more radiation means more matter. In this way, we can identify ‘hotspots’, where higher levels of dark matter may be concentrated.

We already know about photons, electrons, quarks and many other particles, but there may be plenty of others waiting to be discovered. One or more of these unidentified particles could be responsible for effects – like gravitational lensing and the ‘spider’s web’ – that we associate with dark matter.

Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe.

Dark matter might be an undiscovered particle, or it might be nothing at all. Some scientists believe that the effects we associate with dark matter are actually caused by gravity.

Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot.

It’s possible that our existing theory of gravity is flawed, and that the effects we attribute to dark matter could simply be a quirk of gravity that we don’t yet understand.

The number came out 10120 times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.

So is dark matter a particle or a consequence of gravity? Well, we don’t know, and more research is needed before either theory can be proved true or false.

Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy

If dark matter particles exist, then they should occasionally collide with one another: an interaction that produces radiation. And so spacecraft have been fitted with advanced detectors to search for signs of this radiation. Some interesting results have cropped up, but the search for hard evidence is still on.

Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.

If hard evidence for dark matter is ever found, it will be in the form of very small, subtle effects. And so, in order to detect these tiny signals, scientists have developed dark matter laboratories deep underground, far from the effects of overpowering background noise.

By fitting a theoretical model of the composition of the universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter. What is dark matter?

In the UK, there’s Boulby Underground Laboratory: a dark matter lab in North Yorkshire, located over 1,000 metres below the Earth’s surface. And other similar facilities exist across the globe, from Canada to Australia.

it is dark, meaning that it is not in the form of stars and planets that we see.

There’s still a whole lot we don’t know when it comes to dark matter, and some mysteries might never be unravelled. But as technology advances, we are edging closer towards the truth.

Observations show that there is far too little visible matter in the universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons.