Q.  Where is most of the Universe's mass?

A.  About 90% of it is in the form of matter in interstellar space, the "Oort Soup".

Q.  And the rest?

A.  This is in the form of stars, their solar systems, black holes, and galactic cores.

Q.  Why do we know relatively little about the Oort Soup, if it predominates?

A.  Because most of our observations are based on the light given out by the non-Oort components. The Oort components are too cold to give out visible light, and too distant from stars to reflect much light.

Q.  So how do we know about the Oort stuff?

A.  It was originally called "Dark Matter", and has been detected from its effects on the rotation of galaxies.

Q.  Where is there more detail on this?

A.  See P2: The Oort Soup as the real origin of Cosmic Microwave Background Radiation.