Given a scientific question, generate a correct answer to it.

Example Input: Normal blood is comprised of nearly half erythrocytes, which is another word for what cells?
Example Output: red blood cells

Example Input: Gravitational force on a large scale dominates interactions between large objects because it is always what?
Example Output: attractive

Example Input: The motion of stars in galaxies imply that there is about 10 times as much what as in the luminous objects we can see?
Example Output:
mass