Dark matter binds all large objects together, without it stars and galaxies might never have formed.

We know what properties it should have, but have never actually seen it.

With science if a theory can predict everything you see it can be generally accepted. However, this does not mean we are not searching for evidence.

The evidence of dark matter.

From what we have observed the rotation of galaxies is too fast for them to stay together for long. They would have flown apart long ago without some mass that we can not see holding them together. Especially the outer edges. Gravitational lensing (which causes light behind galaxies to warp) is statistically measurable, meaning we get accurate results on how much it is warping light. This means it is a way to easily measure the mass of galaxies (or at least confirm it), a lot of mass is missing. Even in our own solar system we have had trouble with dark matter, the voyager space crafts and their famous "voyager anomaly " which was a slowdown of the space crafts that we thought should not have happened. We measured the gravity of the sun from looking at it, which missed the dark matter completely. This means missing mass must exist, which we have simply named dark matter.

The importance of dark matter.

Without dark energy the universe wouldn't have formed anywhere close to what it has now. Dark matter does not interact with the electromagentic spectrum, which means the massive amount of light would not have effected it. That means that it was able to clump together more easily, which means that it eventually is what gave birth to galaxies. It likely helps in the formation of stars. Gas clouds need to be extremely cold to be able to collected together and eventually form stars. Dark matter can speed up this process by making specific parts of the cloud more dense. It is estimated that the universe is 26.8% dark matter, because of how much gravity is has compared to regular matter.
It also helped combat dark energy which allowed galaxies to form, but is sadly not enough and the expansion of the universe is accelerating.
We know very little about it outside of its effects, only time will tell the full story.



More reading:https://en.wikipedia.org/wiki/Dark_matter
image sources: www.nasa.gov/images/content/214434main_M33_UVOT.jpg
svs.gsfc.nasa.gov/vis/a010000/a010900/a010943/Dark_Matter_pie_chart__Still_1.jpg