Dark matter has been postulated by scientists to explain all the movement and behavior that they cannot explain in the Universe. Insert dark matter, and it all fits, or so they are currently postulating. In fact, dark matter does not explain it all, or even explain it very well, but this is the current fad. Why is dark matter termed dark? Well, because folks can't see it, or measure it, or capture it, and thus it is dark, hiding, as it were. Well, matter does not hide. It has no reason to do so, being without a sneaky motive. It is not dark matter that is gluing everything together in some strange, immeasurable way, it is tiny matter, in fact, which the scientists of Earth have yet to see because it is too little for their eyes and their instruments as of yet.
The particles of tiny matter, like bugs, are more numerous as they get smaller. Humans are always horrified to see how many mites there are in a speck of dust. Mites are everywhere, and should one have the guts to count, the number of mites in the room would be vastly greater than the number of silverfish or cockroaches. And, of course, the number of bugs in the room vastly outweighs the number of humans. Such is the situation in the Universe. Man saw the planets, e.g. humans in the room, and registered their personalities. This happened early on. Then man become aware of the energy that composes the solar wind, and light rays that come from distant galaxies, e.g. visible bugs in the room, and registered their personalities. This happened recently. However, humans are not yet aware of the galactic tiny stuff, e.g. mites, and thus don't have a very important piece of the puzzle in place.
Before humans became aware of the planets, and their relationship to the Sun, there were all kinds of strange explanations for their motions. Since the Flat Earth Society is still among you, we need not go into detail. Before humans became aware of the energy paths in the galaxy, there also were all kind of strange explanations for what seemed to be the erratic nature of the greater Universe, which bobbled and glowed and winked, behavior which was most often ascribed to the gods or one's own misbehavior. Now scientists are dealing with the larger, visible planets behaving toward one another like something else is there. You can assume this to be a massive amount of tiny matter. The stuff the Universe is made of, elemental particles that are not moving, are not clumping, and thus do not form mass or register as energy.
Nothing to get excited about. Just calculate its weight, and think of it as galactic air - something to pass through.