In 2016 Stacy McGough and colleagues studied 150 galaxies similar to our Milky Way. They found that all of them rotated with the speed to be expected of them based on their mass except all of the outer stars of each galaxy were moving faster than they should!  The faster outer stars had been noticed before and excused by suggesting an imaginary halo of dark matter around the galaxies.Thus the requirement is for another source for the ‘extra’ energy. New Gravity with its pull on space has the answer without a special bolt on extra.

In this case as you know every particle of a galaxy even, pulls on space eternally, if 150 Galaxies all exhibit the same unexpected speedy outer stars it follows that it is standard.

Not special.

Gravitational pull from the centre becomes push at the outer edges as moving streams of space hit the outer stars first. Thus there is a tendency to squeeze the galaxy and so release energy for the outer stars to move faster than they would otherwise.

No dark matter just New Gravity showing it’s potential.