What is Dark Matter ?

One of the Dark Matter facts is that 68% of the universe is dark energy, so Dark matter makes up about 27%. The rest – everything on Earth, everything ever observed with all of astronomer’s instruments, all normal matter – adds up to less than 5% of the universe.

Dark matter is classified as cold, warm, or hot according to its velocity. Current models favor a cold Dark Matter scenario, in which structures emerge by gradual accumulation of particles.

There are still many mysteries about Dark Matter that need to be unfolded, but astronomers and physicists have gathered evidence about its presence.

Astronomers have also considered that Dark Matter may be supermassive black holes (single points of infinite mass and gravity formed from the collapse of burned-out massive stars) at the centers of galaxies.

Dark matter is the name we give to all the mass in the universe that remains invisible, and there’s a whole lot of it. Research suggests that about 70% of the universe is composed of dark energy, whilst the remaining 25% is composed of a mysterious substance known as dark matter. We can’t see it, we don’t understand it, but we know it’s out there.

Dark matter is really hard to study, because we have no way of seeing it. This weird substance doesn’t interact with light, so it’s totally invisible.

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.

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.

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.

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.

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.

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.

Dark matter may be complex, elusive and mysterious, but scientists all over the world will continue the hunt – and what they find could eventually transform our very notion of the cosmos.

It means it doesn’t emit any electromagnetic radiation for all we can tell. Astronomers haven’t been able to find neither light visible to the eye, nor radiation in the radio range or x-ray regime, and not at even higher energies either.

What physicists classify as matter must behave like the matter we are made of, at least for what its motion in space and time is concerned. This means in particular dark matter dilutes when it spreads into a larger volume, and causes the same gravitational attraction as ordinary, visible, matter. It is easy to think up “stuff” that does not do this. Dark energy for example does not behave this way.

You will not wake up one day and hear physicists declare it’s not there at all. (Well, you will, but those claims are rare, and those physicists are wrong.) The evidence is overwhelming: Weak gravitational lensing demonstrates that galaxies have a larger gravitational pull than visible matter can produce. Additional matter in galaxies is also necessary to explain why stars in the outer arms of galaxies orbit so quickly around the center. The observed temperature fluctuations in the cosmic microwave background can’t be explained without dark matter, and the structures formed by galaxies wouldn’t come out right without dark matter either. Even if all of this was explained by a modification of gravity rather than an unknown type of matter, it would still have to be possible to formulate this modification of gravity in a way that makes it look pretty much like a new type of matter. And we’d still call it dark matter.

Though she was the first to recognize its relevance. A few decades before Vera Rubin noticed that stars rotate inexplicably fast around the centers of galaxies, Fritz Zwicky pointed out that a swarm of about a thousand galaxies which are bound together by gravity to the “Coma Cluster” also move too quickly. The velocity of the galaxies in a gravitational potential depends on the total mass in this potential, and the too large velocities indicated already that there was more mass than could be seen. However, it wasn’t until Rubin collected her data that it became clear this isn’t a peculiarity of the Coma Cluster, but that dark matter must be present in almost all galaxies and galaxy clusters.

If it did, it would slow down and clump too much and that wouldn’t be in agreement with the data. A particularly vivid example comes from the Bullet Cluster, which actually consists of two clusters of galaxies that have passed through each other. In the Bullet Cluster, one can detect both the distribution of ordinary matter, mostly be emission of x-rays, and the distribution of dark matter, by gravitational lensing. The data demonstrates that the dark matter is dislocated from the visible matter: The dark matter parts of the clusters seem to have passed through each other almost undisturbed, whereas the visible matter was slowed down and its shape was noticeably distorted.

Since dark matter doesn’t interact much with itself and other stuff, it’s the first type of matter to settle down when the universe expands and the first to form structures under its own gravitational pull. It is dark matter that seeds the filaments along which galaxies later form when visible matter falls into the gravitational potential created by the dark matter. If you look at some computer simulation of structure formation, what is shown is almost always the distribution of dark matter, not of visible matter. Visible matter falls into, and hence, is assumed to follow the same distribution at later times.

Dark matter doesn’t only form filaments on supergalactic scales, it also isn’t entirely smoothly distributed within galaxies — at least that’s what the best understood models say. Dark matter doesn’t interact enough to form objects as dense as planets, but it does have ‘halos’ of varying density that move around in galaxies. The dark matter density is generally larger towards the centers of galaxies. Since dark matter doesn’t rotate with the disk of stars we observe, solar systems like our own constantly move into a “wind” of dark matter particles.

The presently most popular explanation for the puzzling observations is some kind of weakly interacting particle that doesn’t interact with light. These particles have to be quite massive to form the observed structures, about as heavy as the heaviest particles we know already. If dark matter particles weren’t heavy enough they wouldn’t clump sufficiently, which is why they are called WIMPs for “Weakly Interacting Massive Particles.” Another candidate is a particle called the axion, which is very light but leaves behind some kind of condensate that fills the universe.

There are other types of candidate particles that have more complex interactions or are heavier, such Wimpzillas and other exotic stuff. Macro dark matter is a type of dark matter that could be accommodated in the standard model; it consists of macroscopically heavy chunks of unknown types of nuclear matter.

Then there are several proposals for how to modify gravity to accommodate the observations, such as MOG, entropic gravity, or bimetric theories. Though very different by motivation, the more observations have to be explained the more similar the explanations through additional particles have become to the explanations through modifying gravity.

We know that dark matter can’t be constituted by dim brown dwarfs or black holes. The main reason this doesn’t work is that we know the total mass dark matter brings into our galaxy, and it’s a lot, about 10 times as much as the visible matter. If that amount of mass was made up from black holes, we should constantly see gravitational lensing events — but we don’t. It also doesn’t quite work with structure formation. And we know that neutrinos, even though weakly interacting, can’t make up dark matter either because they are too light and they wouldn’t clump strongly enough to seed galaxy filaments.