About 27 percent of the universe is made up of the substance we can’t see called ’dark matter’. Scientists are trying to figure out what dark matter is, but each possible answer has its own set of problems. Here are some of the things might be able to explain about dark matter that range from the unlikely to the probably but we still haven't found it yet.
Some scientists aren't sure dark matter really exists. After all, we can't see it directly. What we see are galaxies and galaxy clusters held together more tightly than we'd expect.
Researchers think this clinginess is due to the force exerted by unseen matter. But other scientists want to explain another way by modifying the theory of gravity. They can test these alternatives against observations for instruments such as NASA's wide-field infrared survey telescope and the British Square Kilometer Array.
However, they haven't been able to explain the motion up all objects in space. There must be some unseen element and it's probably dark matter.
Maybe the universe’s extra mass is coming from large objects that are just difficult to observe. They’re called it Massive Compact Halo Objects or MACHOs. These might be black holes, which emit no light or they’re could be cold-faint bodies like brown dwarfs.
Failed stars without enough mass to perform fusion or neutron stars which packed the sun's mass into an object—a quadrillion times smaller—literally, a quadrillion.
Unfortunately, physicists have calculated the amount of dark matter in the universe and MACHOs can't account for it all.
Now that MACHOs is off the table. Dark matter must be made up of more exotic particles which don't normally interact with regular matter. Take neutrinos, for example, tens of thousands of these particles streamed through your body every second without having any impact. Some think dark matter could be a particular type of neutrino called the Sterile Neutrino.
However, sensitive detectors like Fermilab MiniBooNE have been hunting for Sterile Neutrinos and have yet to find any.
Another negative for Sterile Neutrinos is the fact that they are so-called hot particles. Moving it nears lightspeed and hot dark matter fails to explain how galaxies are distributed across the universe.
So, dark matter consists of particles that very rarely interact with regular matter and live significantly slower than the speed of light. This is known as Cold Dark Matter. A particle called the axion could make up this type matter. Unfortunately, it's still hypothetical we haven't directly-observed axion yet.
The ‘Axion Dark Matter Experiment’ or ADMX at the University of Washington is currently on the hunt.
The axion aren't quite as popular as WIMPS, the number one dark matter candidate. WIMP stands for Weakly Interacting Massive Particles. Like axion, they are formed of cold dark matter but they're bigger and more likely to interact with regular particles.
You might think this will make ‘WIMPS’ easier to detect. But, we haven't found anything to prove it.
The cryogenic dark matter search at Fermilab has seen promising WIMPS hints. However, the Large Underground Xenon Experiment or LUX which is the most sensitive detector yet. Failed to reproduce the sightings.
Still, this doesn't mean WIMPS is out for the count. It just means we might be looking in the wrong place.
The experiments mentioned were only studying a limited range of energies. So, we can still hold out for WIMPS. Because the they're off the table, then are really in the dark.