Question: What is dark matter?

Roan Haggar answered on 10 Nov 2020:

The short answer is, we don’t know!
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There’s plenty of evidence in the Universe for dark matter. For example, we can use the speed at which stars move in a galaxy to estimate how much a galaxy weighs, because the mass of a galaxy determines the strength of its gravity. By doing this, we find that the mass of galaxies is much greater than the amount of stuff we can actually see. In fact, we can calculate that about 85% of the matter in the Universe is this ‘dark matter’, that we can’t see or detect.
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We understand quite well how this matter affects the Universe — for example, the gravitational fields that it produces. However, we don’t actually know what it is!
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There’s lots of different theories for what this dark matter is. One idea is that it is made of a new type of particle, called a ‘Weakly Interacting Massive Particle’, or ‘WIMP’. Other ideas include that it may be made of a special type of black hole, called a ‘primordial black hole’. There’s lots of work going on to try and work out what’s responsible for dark matter, so watch this space!