Dark matter is a hypothetical invisible mass thought to be responsible for adding gravity to galaxies and other bodies.
Since the amount of visible material in galaxies can't account for their shapes, distributions, and movements, astrophysicists argue there must be a form of mass yet to be discovered. This mass doesn't appear to affect normal matter significantly in any other way - such as by absorbing or emitting photons - making it completely 'dark'.
In spite of having so little to go on, researchers have continued to narrow down the characteristics of that 'missing matter' and have a few ideas what it might be. If it exists, it would make up 85 percent of the Universe's total material and roughly 25 percent of its energy.
Here are a few possible explanations physicists are currently exploring.
1. Small, weak, and everywhere:
Physicists categorise these under the broad description Weakly Interacting Massive Particles, or WIMPS. These heavy particles wouldn't interact with electromagnetic charges, but could still swap details with normal matter through the weak nuclear force, meaning in theory we might still spot them.
There are other hypothetical particles which could explain it. Low mass, low range particles called axions have never been seen, but might explain why some strange mirror reversals in quantum physics don't quite look as they should. Since they'd break down into things we can see, we just might be able to spot them … if they exist.
2. Big, dark, and few:
Rather than being made of a new kind of material, dark matter could simply be ordinary objects that act in extraordinary ways. Massive astrophysical compact halo objects (MACHOS) are one candidate - heavy, star-like bodies that just don't glow enough.
Unfortunately, there probably aren't enough of them around to make up a full 85 percent of matter in the Universe.
3. Wrong, wrong, wrong:
It's extremely unlikely. But there is simply a chance that we're wrong about how gravity works and what keeps big structures held together. Physicists constantly test and retest Einstein's general relativity, trying to find the smallest discrepancy in case there's some wiggle room that might offer a way to explain dark matter without the need for new kinds of particles or strange objects.
So far, general relativity still looks good.