This is a measure of how much matter makes up a physical object. The standard measure for mass is the kilogram. Something’s mass shows how much the object resists speeding up or slowing down when a force is applied. Imagine pushing (gently) against a sleeping kitten. It will begin to move with very little force. The kitten is small, and it doesn’t have a lot of mass. Now imagine pushing against an elephant. It would take a lot more force to make that animal move. Elephants have a lot more mass — they are made of more matter — than a kitten. More mass doesn’t necessarily mean bigger, though. A bowling ball and a balloon might be the same size, but one has a lot more mass than the other.
An object’s mass also determines how much gravity it has. The sun keeps the Earth in orbit because our star has a lot more mass than the Earth.
In a sentence
Galaxies keep a lot of their mass in clouds of gas.