Invisible But Not Undetectable
The Atacama Large Millimeter Array (ALMA) has captured an image with subtle distortions. That, in and of itself, is not too terribly interesting; however, these distortions hint at a hidden galaxy—a dark, dwarf galaxy made up mostly of dark matter...dark matter than happens to be one of the two missing pieces of our universe.
The image captured by ALMA showed faint red arcs surrounding a galaxy (shown in blue light below) nearly 4 billion light-years away. The distortions of light are believed to have been caused by the gravitational force from the dwarf dark galaxy behind it.
As NASA says, "we are much more certain what dark matter is not than we are what it is." We know that dark matter does not emit or absorb light, making it invisible to us as of now. And while we have no known methods of seeing dark matter, all matter has gravitational force, which has an effect on surrounding matter (including light). As predicted by Einstein’s theory of general relativity, the gravitational forces of objects alter light and cause a lensing effect, called gravitational lensing.
This gives clues about surrounding galaxies, even when they are far away or, in this case, invisible.
"Discrepancies" No More
"We can find these invisible objects in the same way that you can see rain droplets on a window. You know they are there because they distort the image of the background objects,” Stanford University astronomer Yashar Hezaveh explained.
The research implies that we may have not been seeing majority of dwarf galaxies because they are made up mostly of dark matter.
For nearly two decades, researchers have been seeing similar distortions but brushed them off as “discrepancies.” This discovery could explain those discrepancies and open up possibilities for ALMA to find similar objects for comparison as well as help astronomers find out more about dark matter.