There are multiple theorys regading our universe, 1 of them is the expanding universe, but there is 2 others where we dotn expand but collide with Andromeda galaxy. But by that time we dont excist any longer because our sun will evaporate our own solarsystem, making life therefore not possible any longer. Dark matter is something we dont truly understand yet, all we know is, ist there. We cant really see or touch it, but we know something is out there. We just dont have the Technology yet to tell what exactly it is. Ist an interesting theme for sure.