What is the mass of the Universe? Also how can you prove that this is the true mass of the Universe?
As nobody knows the size of the universe, one cannot really talk about the mass of the universe, though one can talk about the mass of the observable universe. What is normally sought after is the density of matter in the universe (which is the mass per unit volume). This is what is important in determining the fate of the universe: whether it will collapse one day or whether it will continue expanding forever.
The density of matter in the universe can be measured by various means, which are too technical to go into at this point: people measure the density by studying the fluctuations in the Cosmic Microwave Background, superclusters, Big Bang nucleosynthesis, etc.
According to these studies, the density of matter in the universe is about 3 x 10^{-30} g/cm^{3}, which means that it is 300 billion billion billion times less dense than water. Note that this includes the contribution of dark matter and so the density of luminous matter (that we see as stars and galaxies) is only about one-tenth of the figure given above.
Now, the size of the observable universe is about 14 billion light years, and using the above value of density gives you a mass (dark and luminous matter) of about 3 x 10^{55} g, which is roughly 25 billion galaxies the size of the Milky Way.
This page was last updated June 27, 2015.