Yes
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No, Einstein did not know about dark matter during his lifetime. The concept of dark matter was not proposed until the 1930s, after Einstein's major contributions to the field of physics.
The only similarity we know of is that they all seem to interact with gravity. Aside from that, scientists do not know what dark matter is.
We don't really know very much about dark matter, so most of its properties (like how it feels) are not known.
Dark matter is matter that does not interact via the electromagnetic force. We don't really know what dark matter is, exactly; there are several possibilities. We know that it interacts via gravity, meaning it has mass. It may, or may not, or SOME of it may, interact via the weak force as well.
We do not know as we have not found any dark matter to examine. The only way we detect it and know it exists is due to its gravitational attraction of the ordinary matter we can see. One speculation when neutrinos were discovered to have tiny nonzero masses was that dark matter might be neutrinos. Another speculation is that dark matter is only ordinary matter, but its in another separate universe in a shared higher dimensional spacetime. Nobody knows.
I dont know correctly but it may be most of the galaxy I think so! In reality we don't 'know' but our best understanding is; Ordinary matter: ~4.5 %, dark matter: 23%, dark energy: 72%. Reputedly 80% of this 23% dark matter is cold dark matter and 20% is hot dark matter. It has been said that "dark matter.... makes up more than 80% of the matter of the universe." but that is a common misunderstanding; But we MAY say; up to 96% of the mass-energy in the universe is 'dark'. We don't know what dark matter is, but in fact it only means it does not have an easily detectable 'electromagnetic cross section' so it could be electrons/ions plasma, which has a refractive index of 1.