That which occupies space and possesses rest mass, especially as distinct from energy is called as matter. Matter is present in its various forms through which we can see, observe, and study it. We see matter in our everyday lives and also when we look up towards the skies, we see it in form of stars, galaxies etc. But, not all of the sky is filled with stars, we also see empty space in between. Which actually is filled up with dark energy, neutrinos, and the mysterious dark matter. As its name suggests, you can’t see or detect it. It can’t be detected directly but it affects rest of the matter (basically its observations) through which it can be studied indirectly, for example, gravitational lensing. It can also be traced in cosmic background radiation and large-scale structure of the universe. Though some scientists have alternate theories to explain above phenomenon, instead of giving its credit to hypothetical dark matter, but science community has vastly accepted this hypothesis.
Galaxies are spinning so fast that they could have been ripped apart but it doesn’t happen, something is keeping them together. Ordinary matter is just 4.9% of the total mass of the universe. Dark matter makes up 98% percent of all matter in the universe and 26.8% of total mass of the universe while 68.3% of the total mass is occupied by dark energy. Thus, it’s helpful in modeling the structure and formation of the galaxies than just studying the cosmos through electromagnetic observations. Direct observations are still not possible.
What could it possibly be made up of ? A widely accepted hypothesis suggests that it is made up of weakly interacting high energy particles which interact by means of gravity and weak forces. So, to detect and study it, non-gravitational detection methods are being developed.
Can you imagine a galaxy made up entirely of dark matter? As put forward by astronomers in 2016, they have possibly found a galaxy made up almost entirely of dark matter. Dragonfly 44 is an ultra diffused galaxy which entirely consists of dark matter with almost no stars or galactic structure. An ultra diffused galaxy is the one which may have mass comparable to Milky Way but with only 1 % star count. You can find some old stars there because of the absence of any new formation as there is an absence of star-forming fuel or gas.
One of the most reliable ways to detect dark matter is through gravitational lensing, especially by observing galaxy clusters. It produces weak and strong lensing, you can see that in the images below:
There is one more way which points towards some “happenings” in the dark matter. While studying data from European Space Agency’s XMM-Newton spacecraft, astronomers possibly found an X-Ray spike coming not from the visible matter but from the dark matter, in the regions pointing towards the Andromeda galaxy and Perseus galaxy cluster. Below is the video:
As dark matter occupies most of the universe, being able to detect and studying it will unravel many mysteries.Experiments at the Large Hadron Collider (LHC) may provide more direct clues about dark matter.