what is Dark matter in space ?

Hubble Space Telescope observations of galaxies (gold) and their movements allowed scientists to calculate where the dark matter (blue) lies. (Credit: J.-P. Kneib/ESA/NASA)

We don't actually know what dark matter is .we can't directly detect it .what we do know is that the universe doesn't behave entirely as it should if we apply our current physics to what we can directly observe.
Dark matter research is unsettling .scientists were unnerved when they first noticed that galaxies don't rotate by same physics as a spinning plate .The stars at galaxy's edge rotate faster than expected . And their motion can only be explained by a lot of invisible matter that we can't see.
scientists have built incredibly sensitive ,bizarre instruments to look for them .there include vats of liquid xenon stored miles underground , and telescope looking for the dark matter particles decaying into things we can see and measure , like gamma rays . It includes the harge Hadron collider, one of the most expensive science experiments ever built .But they haven't found them.

Researchers hope the XENON experiment will directly detect dark matter particles. (Credit: XENON)

The Large Hadron Collider is looking for dark matter candidates. (Credit: CERN)

Researchers  consolidated the possibilities into two main camps.
MACHOs ( Massive Compact Halo Objects )
WIMPs ( Weakly Interacting Massive Particles )

Brown dwarfs are an example of a MACHO, but they don’t exist in large enough numbers to solve the dark matter problem. (Credit: NASA/JPL-Caltech)

MACHOs are the less exotic possibility. We don't glow or reflect light terribly well ,so it's reasonable to suggest that space and galaxies contain lots of stuffs - Planets ,Stars not quite big enough to turn on and light up, world -gobbling space worms that we simply can't see because they are literally dark and we don't have a big enough flashlight.Except we can detect some of those objects out there because they are so massive that they bend light around them. we would need to see these light -bending gravitational lenses everywhere and we don't , even when we look very hard .


So the astrophysics community mostly moved on to WIMPs. Rather than big objects ,may be the universe is full of little things we can't see These would be swarms of objects like atoms that just don't reflect or absorb light or any other kind of electromagnetic energy.
We can't figure out how there are enough of them to make up the 84 percent of universe's matter that we can't see.
So dark matter is different object we haven't observed at all yet, something called a Neutralino.


ref : www.discovermagazine.com