For a long time, astronomers thought that almost all of the Universe was made of regular matter (called "baryonic matter". It is the same kind of matter that makes up everything on earth and all the stars (protons, neutrons and electrons)). However, they have now found out that this baryonic matter was only a small fraction of the matter in the Universe.
By measuring the motion of galaxies, and of stars inside galaxies, astronomers have been able to determine that there exists also some matter that we cannot see. They know that because even though they cannot see it, they measure the effects of their gravity on these stars and planets. This is what we called "dark matter".
There is also something that makes up most of the Universe and that we refer to as "dark energy". The nature of this dark energy is completely unknown, but we know it behaves quite differently from regular matter. It is believed it has an opposite effect to gravity pushing everything apart and thus contributing to the expansion of the Universe.
Very recent observations of the Cosmic Microwave Background have revealed the relative fractions of each of these components. The best estimates now say that the Universe is made of 4% regular baryonic matter, 23% dark matter and 73% dark energy.
This page was last updated June 27, 2015.