What Is Dark Matter?

Dark matter is probably the part of this whole topic of the universe that fascinates me most – and that’s hard to do. There are so many awe-inspiring things to look at and learn but we know so little about dark matter that it kind of gives it an edge, in my eyes.

So what do we know about dark matter? Well it is “inferred to exist from gravitational effects on visible matter and background radiation”, to me that says “something is acting on the stuff we can see, like the planets and stars, and we can see some background radiation from it.” That’s how its existence came to be hypothesized. There were discrepancies between the measurements of the mass of galaxies, galaxy clusters and the entire universe based on the measurements for visible “luminous” matter (stars etc.).

The mind boggling thing to me is that according to observations under the Friedmann equations and the FLRW metric dark matter makes up for 23% of the mass-energy density of the observable universe. When you compare this to the 4.6% measurement of ordinary matter you can get from these figures that dark matter in fact makes up 80% of matter in the whole of the universe! 80%! Space is HUGE!

A lot of the evidence for dark matter comes from the study of the motions of galaxies. Many of these appear to be fairly uniform, so by using the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. However, when this is experimentally the total kinetic energy is found to be much greater: specifically when assuming the gravitational mass is due to only the visible matter of the galaxy, stars far from the centre of galaxies have much higher velocities than predicted by the virial theorem.

From all this we evidence that there is some kind of dark matter, but what is it? Well, this is still unknown. A substantial body of evidence indicates that it cannot be something called baryonic matter, (i.e., protons and neutrons). The more favoured theory is that dark matter is mostly composed of something referred to as “exotic particles” which were formed when the universe was a fraction of a second old. These particles, which would require an extension of the so-called Standard Model of elementary particle physics, could be “Weakly Interacting Massive Particles (WIMPs), or axions, or sterile neutrinos. A typical WIMP is predicted to be at least 100 times as massive as a hydrogen atom.

What’s more known about dark matter is what it’s not likely to be, such as black holes (there are too few to justify the difference in mass for a start).

The research continues as we keep trying to work out and understand the fabric that holds our universe together. The wonder in the universe isn’t just kept for the stars that take all the glory but for the “glue” that keeps it all together too.

Source by S L Hammond

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