What is dark matter?

Hello, Liquid Astatine!

Dark matter is matter which neither emits nor reflects electromagnetic radiation, but its existence and positions are proven by its gravity acting upon regular matter.

Dark matter might be worlds(galaxies) based on antimatter or hyperlightspeedgalaxies where the time goes backwards so that they are invisible for us. If that was true than they should have an organizing influence on proceedings going on in the visible universe - as much as proceedings going on in the visible universe would influence those proceedings in the hyperlightspeedgalaxies. They would be in a kind of instable balance with the visible part of the universe.

A very daring theory. We still don't know what it is.

Still unknown. It is detected by its gravitational influence but it does not react with light so it can't be seen -- hence the name "dark" matter. Theories abound as to what it is. One theory suggests it is non-baryonic, meaning its atoms are not made from protons or neutrons, but something strange and exotic. Another theory is that dark matter is huge fields of neutrinos. Neutrinos do not react with ordinary matter, so the neutrino is a good candidate. Another theory is that dark matter IS baryonic but undetectable because it is not emitting light. For example, brown dwarf stars, lots and lots of brown dwarf stars, that can't be detected because they are too dim, neverthless could make up for a lot of the unexplained mass. None of these theories is really very helpful and all of them have their own sets of problems.

dark matter is matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter.