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Asked by Logan F on 3 Oct 2021.
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Jacob Maresca answered on 10 Sep 2021:
The universe contains three key ingredients:
Regular matter (the atoms and molecules that we are made of, along with the planets and stars etc).
Dark matter (a mysterious invisible matter that helps to hold galaxies together)
Dark energy (another mysterious ingredient that provides the energy to expand the universe)
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Giles Strong answered on 10 Sep 2021:
Build on Jacob’s answer:
Only about 5% of the Universe is the regular matter we are used to. The majority of what the Universe is made of is still not understood, and thanks to various experiments and measurements, we know that we don’t know.
Dark matter and dark energy are two of the greatest challenges we seek to solve, but hopefully, one day, through theoretical prediction and experimental discovery we will eventually be able to understand what the Universe is really made of. -
Andrew Hone answered on 14 Sep 2021:
Matter, dark matter and dark energy (see the other answers).
However, the only part that we understand really well is the matter, which seems to be only about 5% of what we observe.
The part that we do understand (matter) is described very accurately at high energy by quantum field theories of a particular type, called gauge field theories, which combine into something called the Standard Model of particle physics, explaining the interactions between fermionic particles (electrons, muons, tau particles and their associated neutrinos, and quarks) that are mediated by bosonic particles (photons, W, Z, gluons and Higgs). All other ordinary matter is, as far as we know, built from bound states of these particles – for instance, the protons and neutrons which are found in the nuclei of atoms are each made from a combination of three quarks. The Standard Model explains the electromagnetic, weak and strong nuclear forces. It is also has Einstein’s Special Theory of Relativity built into it, so it is consistent for all velocities up to the speed of light (a bit less than 3 x 10^8 m/s).
The elephant in the room is the force of gravity, which causes attraction between all massive objects. This is described extremely accurately across cosmological scales by Einstein’s General Theory of Relativity. However, we do not know how to construct a correct quantum version of General Relativity, which is required if we want to describe extreme situations where there are very strong gravitational fields interacting across very small distances (like the centre of a black hole, or the origin of the universe at the Big Bang). There are various candidate ideas for quantum gravity, but none of them have yet made sensible predictions that can be tested with experiments.
Dark matter and dark energy are both constituents of an inflationary theory of cosmology, based on General Relativity, and they are necessary ingredients to explain the observed motion of the distant stars and galaxies and the way they are expanding away from us. The matter that we can see (because it emits electromagnetic waves, including visible light) is not sufficient to explain what we observe. Dark matter is some extra “stuff” that has mass so attracts ordinary matter via gravity, but doesn’t otherwise interact with it, while dark energy is a bit like an extra field filling all of space, which causes the expansion of the universe to accelerate. It is still the greatest mystery what dark matter and dark energy are!
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