How do we know there is dark Matter in the Universe while its invisible?
It has a gravitational effect. Dark Matter is posited to explain observations where we cannot observe enough matter to cause the gravitational motions we observe. A classic example is the rotation curves of galaxies. We can detect the rotational speed at different distances from the centre of a galaxy by observing the shift in features in the spectrum of light coming from that star or that part of the galaxy. Based on the matter we can see (using observations across the electromagnetic spectrum) and our understanding of gravity, we expect the speed to rise to a peak and then decay away as you move outwards from the centre of the galaxy. However, what we observe is that the rotational speed maintains a fairly constant level out to the edges of the galaxy. This implies that there is more gavitating matter in the outer parts of the galaxy than we can observe. In a general sense “dark” matter can be ordinary matter (atoms). In the 19th century it was apparent that Uranus did not move exact
