Astronomers have proposed the existence of an unexplained force that is causing the expansion of the universe to accelerate. what do they call this force?
dark matter
dark space
dark stars
dark energy
dark holes

4. Dark Energy. This is the mysterious force causing the expansion of space. No one has ever seen this force.

Answer;

Dark matter

Explanation;Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart. One explanation for dark energy is that it is a property of space.Space has amazing properties, many of which are just beginning to be understood. Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field. Dark matter makes up about 27%.

2. they have negligible mass.