As if the above oddities are not enough, we are faced with two great mysteries that we don't know what they are. One of them is dark matter that we cannot see because it does not reflect light. The other is the dark energy that is responsible for accelerating and expanding the universe.
We don't know what dark matter is, but we know that the celestial bodies in the galaxies wouldn't stay together if it wasn't, because gravitational force alone isn't strong enough to form a galaxy. So, what holds the galaxy contents together is this material that we cannot see.
Similarly, we don't know why dark energy exists, but we know that it acts as a kind of negative gravity, stretching and expanding the universe.
In the meantime, although the matter and energy in the galaxies are able to remain together, the space-time tissue between them expands and the neighboring galaxies are rapidly distancing from each other.
