Submitted by Sci Fi Guy on
Image by Alexander Antropov from http://Pixabay .com
Our universe is almost completely invisible to us.
As much as 96% of the universe is comprised of dark matter and dark energy, and therefore only 4% is what we can see. This is known as visible matter. Visible matter consists of everything you can see or touch, including all the stars, planets, and galaxies. All of these emit or reflect light, allowing us to see them.
Dark matter and dark energy do not emit light, and so we can’t see them through any part of the light spectrum, not even X-rays or radio waves. This invisibility makes dark matter and dark energy difficult to detect, and even more difficult to know what they actually are.
What do we know about dark matter?
Dark matter makes up about 27% of all the matter in the universe. We know it exists, in some form, because we can detect some missing mass in the universe. As we can’t see it, we’ve named it dark matter. We know more about what dark matter isn’t, than what it is.
We know it isn’t made of the same type of particles as normal matter, called baryons, because we would be able to detect these. We know it isn’t antimatter because we do not see the gamma rays that are produced when antimatter collides with normal matter.
Once we rule out what dark matter isn’t, we can start to look at what dark matter candidates are left. It is very likely that dark matter isn’t comprised of the known particles, so it must be made of new and different particles that we have yet to discover. Many underground detectors are searching for so-called WIMPs (Weakly Interacting Massive Particles), hoping to catch a rare dark matter collision in enormous vats of dense liquid. So far, no luck.
There is also still a chance that dark matter is made from normal matter if it’s comprised of many, dim brown dwarf stars or dense chunks of heavy elements. These so-called MACHOs (Massive Astrophysical Compact Halo Objects) would circle galaxies undetected, providing the necessary extra mass. On-going investigations and detectors will determine which one of these dark matter candidates is the most viable option.
If we can’t see dark matter, how do we know it’s there?
Dark matter cannot be directly detected through light, but it can be indirectly detected by the gravitational effect it has on the universe.
This is called gravitational lensing. The mass and gravity of dark matter bends the light from galaxies and stars. We can detect this bending of light and determine where dark matter is located. Gravitational lensing is one of the best tools with which to find dark matter.
But dark matter was first found by an American astronomer, Vera Rubin, who was studying how fast galaxies rotate. She added up all the visible mass in a galaxy to estimate how much total gravity it had. Based on this, her physics equations predicted that the stars and gas on the outer edges of a galaxy should rotate slower than the stuff closer to the galaxy core.
However, observations revealed that these edge stars and gas do not rotate slower at all. In fact, they can even have a higher rotation speed!
Vera Rubin speculated that there must be missing matter to reconcile the difference between theory and observation. It is this missing matter which we call dark matter, and it’s yet to be found.
How are we searching for dark matter?
There are three types of experiments which are searching for dark matter: indirect searches, collider searches, and direct searches.
Indirect searches take place in space by telescopes such as the Fermi Large Area Telescope. This telescope detects the by-products of the collision of dark matter particles, in this case, gamma rays.
While this method looks at what dark matter produces, collider searches, undertaken by the Large Hadron Collider, tries to produce dark matter through the collision of normal matter particles.
The third category of direct searches consists of underground experiments such as XENON in Italy or CDMS in the United States, which aim to detect energy that is left over from the collision of normal matter particles and dark matter particles.
Dark Matter Day
This on-going hunt for dark matter is celebrated on Dark Matter Day every 31 October. Find an event near you to find about more about the mysteries of dark matter and how teams around the world are involved in the hunt.
It’s both astonishing and exciting to see just how little we know about the universe.
Tori Tasker
Read more @ https://spacecentre.co.uk/blog-post/whats-in-the-dark/