I'm wondering what properties of Dark Matter are known for certain. Such as: How much does it collide with itself, and with ordinary matter? What does ##\sigma/m\le 7cm^2/g## mean (and why is the estimate rising in more recent studies)? How does it compare with ordinary matter, e.g. liquid water or galaxy filled with stars? Is there any evidence that DM does interact with ordinary matter via weak interaction? Is lack of DM particles in LHC a constraint on this? Can it have more than 1 component? How does its density distribution compare to ordinary matter? How much spherical are galactic halos, and how much bigger are they compared to ordinary matter? How much of it is out there? Is the 5:1 ratio to ordinary matter still the best estimate? If it's made of massive dark objects, what mass ranges are excluded? If it's made of particles, what mass ranges are excluded? Is anything known about its temperature? How does its amount change since the Big Bang? I'd appreciate layman answers I can try to read some publications, but the amount of literature is overwhelming and most of it seems to be outdated.