Dark Matter. Something Is Out There.
Something is lurking among stars. A monstrous mass, larger than collective mass of anything we can see and measure, that cannot be directly observed because it does not absorb, emit or reflect light. Yet for all we know it holds galaxies together, prevents them from falling apart. What else does it do? What exactly is it? Nobody knows.
Galaxies in our universe seem to be doing something they should not be: they are rotating with such speed that the gravity generated by their observable matter could not possibly hold them together. In other words, if gravity was the sole and only force holding the stars clustered (forming a galaxy) then by all calculations galaxies as we know them should have torn themselves apart long ago because the holding force of gravity is simply too weak to keep them in place (imagine swinging an anvil with a thread attached to it. The holding force generated by such thread would represent gravity, and the anvil would be the mass of stars in a galaxy). The same is true of groups of galaxies called clusters, which leads scientists to believe that something we cannot see is at work. They think something we have yet to detect directly is giving these galaxies extra mass, generating the extra gravity they need to stay intact. This strange and unknown matter was called “dark matter” since it is not visible.
Dark matter
Unlike normal matter, dark matter does not interact with the electromagnetic force. This means it does not absorb, reflect or emit light, making it extremely hard to spot. In fact, researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter.
Dark matter seems to outweigh visible matter roughly six to one, making up about 26% of all the matter in the universe. Here’s a sobering fact: The matter we know and that makes up all stars and galaxies only accounts for 4% of the content of the universe! But what is dark matter? One idea is that it could contain “supersymmetric particles” – hypothesized particles that are partners to those already known in the Standard Model. Experiments at the Large Hadron Collider (LHC) may provide more direct clues about dark matter.
Many theories say the dark matter particles would be light enough to be produced at the LHC. If they were created at the LHC, they would escape through the detectors unnoticed. However, they would carry away energy and momentum, so physicists could infer their existence from the amount of energy and momentum “missing” after a collision. Dark matter candidates arise frequently in theories that suggest physics beyond the Standard Model, such as supersymmetry and extra dimensions. One theory suggests the existence of a “Hidden Valley”, a parallel world made of dark matter having very little in common with matter we know. If one of these theories proved to be true, it could help scientists gain a better understanding of the composition of our universe and, in particular, how galaxies hold together.
Dark energy
Dark energy makes up approximately 70% of the universe and appears to be associated with the vacuum in space. It is distributed evenly throughout the universe, not only in space but also in time – in other words, its effect is not diluted as the universe expands. The even distribution means that dark energy does not have any local gravitational effects, but rather a global effect on the universe as a whole. This leads to a repulsive force, which tends to accelerate the expansion of the universe. The rate of expansion and its acceleration can be measured by observations based on the Hubble law. These measurements, together with other scientific data, have confirmed the existence of dark energy and provide an estimate of just how much of this mysterious substance exists.
Source: CERN
More on dark matter and dark energy: Wiki, NASA
10 Awesome Ancient Wonders That Remain A Mystery - The Abandoned World: 
