Dark Matter
The phrase "dark matter" has certainly crossed your lips before, but if you're not sure what it means, you're welcome to the club. Even if science struggles with it, it provides many answers to fundamental issues about the nature of the universe and can currently be used to explain a wide range of cosmic events. Based on our data, the motion of galaxies is illogical. Galaxies look to be far larger than they actually are, and they move as such. Any galaxy must be held together by a hidden source of mass, and dark matter is that source.
The name "dark matter" refers to a substance that does not reflect, absorb, or emit electromagnetic. Since it is virtually invisible, it is only an idea. However, what it does is emit gravity, which keeps the cosmos in place. There is a lot of it as well. In fact, it makes up about 80% of the universe's total mass.