Answer:
There are clusters of galaxies, and the space between them is filled with hot gas, which can only be seen as X-rays or gamma rays.
When measuring how much gas there is between those galaxies, scientists realized that the material should be around five times what can be detected. That undetected matter is known as dark matter.
"Just to clarify- for those that do not know" (I will continue in the comments)
It’s D use it because it helps you remember
