There's not enough matter in the cosmos. For example galaxies seems to contain to little matter for them to hold together. And stars rotate around the galaxies to fast. There should be some more matter, matter which doesn't send out any light or lets light bounce of them so that we can detect them, to hold the galaxies together by its gravity and accelerate stars by its gravity. Infact it seems like we only can see about 95% of the matter in the unverse. Other reasons why we need to know how much matter there is in the universe, is because if there is to little the universe will never stop expanding and vice versa. And its important to know if we want to know how galaxies form. And so on.

The so called 'dark matter'(it doesn't emmit any light) can be explaned in a number of ways. It could be particles:


Stands for MAssive Compact Halo Objects, this family should be made up by normal matter partilces(protons, neutrons, electrons) and should be objects like; white or brown dwarfs or black holes and such. . They seems to exist in big halos around galaxies. Although MACHOS cant be seen, they can still be indirectly detected useing something called gravitational lensing. Gravity bends light, and if you have a large object precent between you and another object gravity can bend the lightrays from their normal paths so the rays which normaly wouldn't reach you, will. And you'll see multiple images of that object:

The white lines(light rays) are going from the object and continueing out into space gets bend when they pass through the large object. Then they instead reach the observer(us) and the dotted lines are were we persive the light rays as comming from. So we see multiple images of the same object.

Stands for Weakly Interacting Massive Particles, they only exist in theory so far and constitute a whole new kind of particles. WIMPS are thought too make up allot of the dark matter. Cold dark matter WIPS are intersting because they would be heavy and move slower then light, so matter might have gathered around WIMPS and then formed galaxies, so in the center of galaxies there might bee some dark matter WIMPS. Allthough they are hard to detect because they only interact weakly with ordinary matter(there by the name) so they're pretty hard, but not impossible, to detect since only a few of these particles would interact with matter and therefor with our measureing devices.


All though not as cool name as the other two, but these have been proven to exist. There are incredible many neutrinos, and although they have a very small mass they should contribute significantly to the dark matter in the universe.


There is two "kinds" of dark matter, hot(the particles in it moves near the speed of light) and cold(the particles move very slow) the state of the dark matter is not importat for knowing if the universe will expand forever, but it lets us know how galaxies formed in the biggining. If there only was cold dark matter it first would have formed galaxies and then they would have come together as clusters, but if that's the case there would not be any big voids and long structures that has been observed. On the other hand if there only was hot dark matter, then clusters would have come first and then forming galaxies, but of course there's a problem with this too, then galaxies would have been formed to late and we wouldn't be able to observe things as quasars. However we don't have to worrie about this since the latest obsevations and calculatons shows that there probebly where both

Resent developments shows us another interesting possibility.
The dark matter could be matter in other parallel universes. parallel universes is an idea which suggest that there exists other universe but we can't see them because the lay in another dimension, another direction. Think of a sheet of papper as being a two dimensional example of our universe. Then we should be two dimensional creatures and therefor only be able to see in two dimenstions and not up. Then if there were other papper sheets stacked upon each other we couldn't see them, even though they're right there, since we can't see up. It's about the same idea with parallel universes.

Now string theory(a new theory which tries to unify general relativity and quantum mechanics) looks at particles not as spots as is the normal view, but as tine strings. These strings can be open or closed. Then, according to string theory, the particles which transmitts the first three forces, strong, weak and electromagnetic force, are all open strings. And open strings would stay on our universe, since they have open ends which can be attach to our universe. But the graviton(particle which transmitts gravity) is, according to string theory, closed loops. So they can infact wonder of from our universe to another.

So dark matter could be matter in another universe. And there's also an explanation why most dark matter seems to exist in the halos around galaxies. Gravity is an attractive force, which means that gravity pulls things closer to itself. So two galaxies in two different universes should actually align. And the major part of the other galaxy in parallell universe should be blocked by the galaxy in our universe, so it should look like all the dark matter existed around the galaxy.

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