Question: What is "Dark Matter" and "Dark Energy"? Is there any progress in this topic?

David Freeborn answered on 22 Jun 2013:

Hi usman100,

The short answer is we really don’t know! Dark matter is a big puzzle for physicists right now, and dark energy is an even bigger one.

Dark matter is very weird indeed. We know that it does interact with gravity, so it has a mass. But we know it can’t be composed of ordinary atoms, because it doesn’t seem to interact with light (otherwise we could see it). But it seems to make up almost a quarter of the Universe.

For a long time, physicists thought dark matter might be Brown Dwarves. These are large masses of hydrogen and helium gas, much bigger than the largest planets like Jupiter and Saturn but too small to become a proper star and start emitting light. Unfortunately, we don’t think this can be the explanation any more, because our telescopes like Hubble should have seen many more of them. There just aren’t enough Brown Dwarves to explain Dark Matter.

A lot of physicists think dark matter might be a new type of particle. The most popular idea is they might be “supersymmetric” particles: heavier cousins of the particles that already exist, but which never or rarely interact with them. We can’t see supersymmetric particles, because to do so we would need to use ordinary particles like the photons which make up light. There are a lot of experiments being built to try to detect supersymmetric particles, and they are trying to make supersymmetric particles at the Large Hadron Collider too. So far, none have been seen. Until then, dark matter will remain very mysterious.

Dark energy ( almost 3/4 of the universe) is even weirder, because it seems to behave like an “anti gravity”. Most heavy objects pull other heavy objects together, and slow down the expansion of the Universe. But Dark energy seems to be speeding up the expansion of the Universe: the expansion is accelerating.

The most popular idea is that Dark Energy is somehow related to the “Quantum Vacuum”. In quantum field theory, nature is able use Heisenberg’s Uncertainty Principle to borrow very small amounts of energy for very brief amounts of time, to create a sea of particles in the vacuum. These create a tiny pressure on everything else, and this has been measured. We think this pressure might be pushing everything apart, and causing dark energy.

The problem is, the pressure physicists have calculated to expect from dark energy is wrong: very wrong. Wrong by 100 orders of magnitude (a 1 followed by 100 zeroes). This has been called the “worst prediction in the history of science”.

So the real answer is, we don’t know! We need a new theory to explain dark energy, and right now nobody knows what these things are!