It refers to what we can't see. We know its there because it exerts a force on something but we just can't see it. Scientists used the term Dark instead of Invisible because to some people the latter means it can be undetected and unaffect the surroundings. The Universe that we know of is made up of 70% Dark Energy, 25% Dark Matter and only 5% Matter. The stars, planets, asteroids etc. are classified as matter. Dark Matter is not Anti-Matter as the former exhibit the same characteristics as normal Matter. Anti-matter operates completely opposite to the way matter operates which Dark Matter doesn't. There is probably Dark Anti-Matter as well but lets not confuse things just now, plenty of time later for that. Dark Matter is grouped into two. The first is WIMP (weakly interacting massive particles) and MACHOS (massive compact halo objects). MACHOS is a very dim object that exists within the HALO of another object. WIMPs are massive objects in space but are exceedingly dark and cannot be detected through light radiation.
Dark Energy is unaffected by gravity and therefore whilst everything else including light protons can be captured by a black hole, Dark Energy isn't. Cornell
In 1933, Swiss astrophysicist Fritz Zwicky, of the California Institute of Technology first proposed the idea of Dark Matter when he tried to calculate the mass of the Coma cluster of galaxies. He realised that the galaxies in the Coma cluster didn't have enough visible matter to hold them together so began the quest for dark matter. He did this by calculating the cluster's total mass based on the motion of the galaxies on the edge. When he compared the results with other galaxies in the cluster, the result was that it was 400x's bigger than what he was expecting. This led him to propose that there was some matter which he could not detect that was having an effect on the mass of the cluster. The Coma Cluster is a cluster of over 1000 galaxies that is within the boundaries of the constellation of Coma Berenices.
In 1998, Scientists discovered that after the Big Bang, the universe expansion began slowing down but then something happened in around 6 Billion years ago that caused the rate of expansion to increase again. The Scientists had been trying to find the rate in which the expansion was slowing down but the opposite was the case. Before then Einstein had proposed that there was something out there that the universe could not be held static unless there was something exerting a force into it.
We have not yet been able to detect Dark Matter or Energy just yet. Scientists are building underground laboratories to capture the particles but so far I have been unaware of any successes. Like black holes, the only proof that they exist is their effect on other celestial bodies. The European team at CERN and the American team at Tevatron, Femilab are conducting experiments by smashing particles at high speed in what may or may not produce evidence of Dark Matter. Scientists are using a variety of methods to discover dark matter from as mentioned particle accelerators to less technical ones in caves under the ground.
Since the the beginning of the universe, the universe has been expanding from the singularity from which it spawned. It is believed that the Universe has not stopped expanding from the moment of its birth some 13.8 billion years ago. It is still expanding now. Scientists have calculated that the universe is expanding at faster than the speed of light. There is a theory that Dark Energy is driving the expansion and its not going to stop any time soon. As energy is finite, it is believed that Dark Energy is finite and once all the energy has been used up, the Universe could stop expanding or even contract on itself. The Universe could if it expanded lead to it breaking up like when you pull a piece of paper in opposite directions, after a while, it will tear or could bounce right the way back to the singularity because gravity has taken control. The reason given for the universe to contract is that Gravity could bring the universe back on itself and pull everything back into the singularity that it was once. Once back as a singularity, it could expand again. We just don't know but the contraction would not happen for millions, billions, trillions of years so nothing to worry about.
There might not actually be Dark Matter or Energy, there could be another explanation for the Coma Cluster but everything at the moment points towards Dark Energy and Matter as the explanation. Until another explanation comes forth, scientists will keep on researching this area. If we do find Dark Matter/Energy, could it lead to new technology, items that we never dreamed of, new ways of travels, maybe wormholes? Until we discover Dark, we will need to keep open this possibility.
In a word, the answer is No. Dark Matter is something that we don't know about or even if it exists. Anti-Matter exists, in simple terms, it is Matter reversed, what do I mean by that. A matter particle with a positive charge is an Electron but Anti-Matter with a negative charge has a Positron. If the two meet, they will annihilate one another. Science Questions
At the beginning of the universe, there was nearly the same amount of matter and anti-matter but all the anti-matter only exists in tiny quantities. Scientists have been able to create anti-matter but they don't last for long. Multiverse.
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