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Asaf Pe'er

Physics for all

What is the universe made of?

To the best of our knowledge, the universe is made of 4 ingredients.

The first is radiation - namely photons. As far as total energy, all the photons (light) in the universe contain a tiny fraction, ~10-5 of the total energy in the universe.

The second is ordinary matter - all the atoms that we are made of, which are the same atoms that planets and stars are made of: namely all the elements in the chemical table, from Hydrogen to Uranium (of course, with different abundances). All the known matter sums to less than 5% of the total energy in the universe.

The third is dark matter. This is a form of matter which nobody really understands. As its name implies, it does not emit (or absorb) any light. However, it does interact with ordinary matter via gravitation. Its existence can therefore be inferred by its gravitational attraction of regular matter. The origin of this matter is unknown, and it is widely believed that it is composed of new elementary particles that have not been detected yet. While we do not feel its influence here on earth, nor indeed in the solar system, it becomes dominant on a scale of a galaxy (and larger). It contains about 27% of the total energy of the universe (recall that E=m c2), namely more than 5 times more than normal matter.

The fourth, and mysterious of all, is the dark energy. (Note the difference between dark energy and dark matter). While gravity always attracts, it turnes out that there exists some mysterious force that causes repulsion of galaxies: distant galaxies not only recede away from us (as discovered by Edwin Hubble), but that they are, in fact accelerating away. The cause of this acceleration is unknown, and inferred as "dark energy" (also "vacuum energy", as it is the energy stored in the vacuum between the galaxies). This is not small at all: energetically, it contains about 68% of the total energy in the universe.

So, the sad fact, is that as both dark matter and dark energy are of yet not understood, we do not really understand the origin of 68+27=95% of the content of the universe! We do understand (up to some level) only about 5% of the universe in which we live...

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