Milky Way’s Dark Companions
A frustration to astronomers these days is how to account for the amount of gravitational effects observed. It appears that something is causing gravity and that something cannot be directly observed. It is being called Dark Matter.
About 85% of the matter in the universe is thought to be in invisible form distinct from atoms and known elementary particles. This invisible stuff is five times as abundant as “normal” matter in the universe at large.
For dark matter comes from our observations of epochs ranging from the first minutes after the Big Bang to the present day, size scales ranging from dwarf galaxies to the whole observable universe, and from a diverse set of astronomical instruments and techniques.
However, all of these observations show only the effects of gravity—we know little about dark matter beyond its gravitational influence. Many extensions to the Standard Model of particle physics predict new particle species that might make up some or all of dark matter.
A better understanding of dark matter could support or refute radically different perspectives on the fundamental nature of matter, energy, and spacetime.
Matter satellites about dark matter. 
The image above from a supercomputer simulation displays as bright clumps the dark matter satellites that can be found around our Milky Way galaxy. The central region corresponds to the luminous matter (gas and stars) of the Milky Way.
 Keith Bechtol, “The Milky Way’s Dark Companions,” Sky and Telescope (133, 3, March 2017), pp. 16-21