Jörg Dietrich explains: Dark Matter

"Most of the matter in the Universe is made of stuff that is very
different from the normal matter that everything in our daily lives
is made of, you, me, tables, chairs, etc. It is different because it only
interacts via its gravity. Dark matter doesn't give off light, it
doesn't absorp or reflect light either. If you had an object made of
dark matter not only could you not see it, you also could not touch
it because it does not interact with you in the way normal matter
does. Your hand would go right through it. Astrophysicists know
that dark matter exists because of its gravity. It holds objects together
that would otherwise fly apart because there is not enough normal
matter to have the required gravity to hold them together."