Dark energy question

Velocity ads somewhat to mass, but mainly near the speed of light
according to Einstein's theories.

A galaxy which rotates seems to have acceleration due to
aligning to a flat plane. Can this acceleration ad mass to
a galaxy? Einstein's theories predict a small mass gain
due to high velocities, but measurements show that as
little as 10 percent of the mass is seen in our galaxy.

I personally assume that mass actually drops with velocity
and dark energy rises, that E=mc2 works well with electromagnetism
but mass itself works in complex relativistic forms.

George