Most of it is invisible
Human beings are short-lived. The matter they are made of, on the contrary, is very long-lived. Human beings – and with them the universe as a whole – are made up of protons, neutrons and electrons, which were created a fraction of second after the Big Bang. That was 13 700 millions years ago. Matter is thus very stable. It remains however very mysterious. The matter that we know seems for instance to be only a small part of the whole matter in the universe. Researchers know this from theoretical considerations and astronomical measurements.
"The mass of our universe consists to 96% of things that we don't understand: dark matter and dark energy", says physicist Rolf Landua. The dark matter emits powerful gravitational forces that can't be understood from the known matter alone. The dark energy explains why the universe expands always faster. The gravitational force in itself should slow down this expansion – but this is precisely not happening.