Of all that exists in the universe, only about 5% consists of what we commonly call matter, ie. atoms, and molecules. The rest we don’t really know what it is. We know that a quarter behaves almost like matter, but we cannot see it – dark matter. The rest is some kind of energy that seems to be linked to the extent of the room itself – dark energy.
If you hold your hand, about six trillions of neutrons flow through it every second. Neutrins are very light elemental particles, but they hardly react at all with ordinary matter, and therefore do no harm. Most of them come from nuclear reactions in the Sun’s interior, but some also come from the birth of the universe in the Great Bang (Big Bang).
There are more stars in the Milky Way than there are people on Earth. The number of stars in the universe is more than the total number of grains of sand on Earth.
Every second there is a supernova explosion somewhere in the universe. A supernova is a dying star that collapses in a powerful explosion, and that, for a short time, shines stronger than an entire galaxy.
Every thousand stars is so heavy that after a supernova explosion it collapses into a black hole. A black hole has such strong gravity that not even light can leave the hole, and we can only see it indirectly by radiation from matter that is about to be sucked into the hole. In the Milky Way, a new black hole is formed about every hundred years and the distance to the nearest black hole is probably “only” thirty light-years.
Gravity may seem to be a fairly strong force but is really very weak. If one compares the electric and gravitational force between two electrons, the electric is 10 ^ 40 (ten thousand trillion trillion trillion) times stronger.
Compared to its extent, the relative distance between two galaxies is much shorter than that between two stars. If Milky Way were as big as an apple, the nearest galaxy would be a meter away, because if the sun were as big as an apple, the nearest star would be a thousand miles away. The Milky Way’s closest galaxy neighbor, Andromeda, is also on a collision course with us. In approx. five billion years it slams!
In the Large Hadron Collider at the particle physics laboratory CERN, you accelerate, among other things. heavy atomic nuclei to almost the speed of light and collide with each other. At the moment of collision, temperatures and particle densities that correspond to the state of the universe occur about a millisecond after the Big Bang.
When it was possible for the first time in 2015 to observe the so-called gravitational waves, these came from a collision of two black holes that occurred 1.3 billion years ago. In the collision, the black holes devoured each other and formed a larger black hole. The energy emitted in the form of gravitational waves in the bang corresponded to about two hundred solar total mass per second.
The last thing you don’t know about the universe is what it looked like before the Big Bang. But don’t be sad, because nobody else knows either. In fact, nobody even knows how the question of what it looked like can be formulated in a sensible way.
