What if 90% of your furniture was invisible? You would have to think of some pretty creative ways to find it. Personally, I would drape sheets over everything in my house, great for Halloween time.
Unfortunately, it would be pretty difficult to cover all of the invisible matter in the universe with blankets, they just aren’t big enough. From what astronomers have discovered, as much as 90% of what makes up the universe is completely “invisible.” In other words, it does not radiate any signatures that can be detected in the electromagnetic spectrum by current technology [1].
The good news is that astronomers have created a few extremely clever ways of guessing the general mass and location of all of this dark matter.
Individual galaxies are always evolving because of the mutual gravitational pull of their galactic neighbors. But we know that the gravity caused by visible mass alone is not enough to stretch across the large distances between them. The gravity added by invisible dark matter is the major factor as we watch many galaxies growing, shrinking, transforming, and colliding. The paths that they take allow astronomers to find the gravitational forces at work on them. From these forces, we can calculate the mass of dark matter that the galaxies must contain to sustain their motions [2].
Another way of searching for dark matter involves examining the x-rays released by large dust clouds. X-rays are only detectable because dark matter between their sources and where they are detected. induce lensing. Gravitational lensing refers to a phenomenon where electromagnetic wave, like x-rays, are pulled slightly towards a large mass which gives a curve to their path. Because the path the rays follow can be approximated, any curves give away the location of large masses, like dark matter. The amount of dark matter can be estimated based on the size and shape of the curve. So far, matter found using this technique accounts for 20 to 30% of a galaxy cluster’s total gravitating mass [3,4].
A similar way to detect dark matter involves spotting rings or arcs around clusters of galaxies called “Einstein rings”. These rings are also caused by gravitational lensing, however, scientists look for waves in the visible light spectrum as opposed to x-rays. In order to form a ring, or even a partial ring, the two objects must be almost perfectly aligned in relation to the observation point. The light from the hidden object is then stretched around the foreground object forming the light ring seen in the image below [5]. How well the ring is defined can be useful in estimating the total mass of the foreground galaxy [6].
Image made up from several images taken with the DECam camera on the Blanco 4m telescope at the Cerro Tololo Observatory in Chile.
It’s important to find out as much about where dark matter is located as possible. Future space missions can take advantage of these bodies of mass to slingshot spacecrafts into deep space, but you don’t want any surprises with billion dollar equipment.
References:
1 http://www.indiana.edu/~geol105/images/gaia_chapter_1/dark_matter_in_the_universe.htm
2 http://www.indiana.edu/~geol105/images/gaia_chapter_1/dark_matter_in_the_universe.htm
3 http://www.indiana.edu/~geol105/images/gaia_chapter_1/dark_matter_in_the_universe.htm
4 https://arxiv.org/pdf/1504.05254v3.pdf
5 https://www.space.com/33095-nearly-perfect-einstein-ring-discovered-image.html
6 http://www.indiana.edu/~geol105/images/gaia_chapter_1/dark_matter_in_the_universe.htm