Dark matter may explain
a. unexpected orbital velocities of stars in galaxies
b. the source of energy that fuels stars
c. the expansion of the universe
d. the origin of life
e. what came before the big bang

A.unexpected orbital velocities of stars in galaxies

Explanation:  

Dark Matter makes up about 80% of matter in the universe. This matter does not emit light and hence, it is called dark matter as it cannot be observed directly.

The effect of the presence of the dark matter is unexpected orbital velocities of stars in galaxies. At the center, the orbital velocities are greater than those at the edge. Had there been no dark matter, the galaxy would have flown apart.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.

pythagoras' theorem

square root (-5^2+9^2)

square root (25+81)

square root (106)

angle is tan^-1(9/5)

neg x, pos y