Dark matter and velocity

Question

We know that as the speed of an object increases, so does its relativistic mass.

But can it be said that dark matter may be due to the increasing relativistic mass of nebulae, stars, galaxies, etc. caused by their high speeds?

Answer 1

The gravitational influence of objects, which is how their masses are estimated, already includes the effects you are talking about.

In fact this is a very minor issue because most of the masses within the universe are not moving at relativistic velocities with respect to each other.

The exception to this is perhaps primordial neutrinos, but even there, the total mass/energy density, that is dominated by their kinetic energy is far too small to account for dark matter.

A further point to consider is that the dark matter required is "cold". That means, by definition, that it has the property of moving non-relativistically.

Answer 2

Relativistic mass becomes important as an increased inertial mass at speeds commensurate with the speed of light.

Dark matter was postulated in order to reconcile observed rotational curves of galaxies with the calculation of expected curves.

Rotation curve of spiral galaxy Messier 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy.

The lower curve is the calculated one .

For relativistic mass to have an effect, the rotational velocities should be close to the velocity of light. From the measured velocities for this typical spiral galaxy we see that there can be no measurable effect, as the velocity is very low with respect to the speed of light which is 299.792.458km/s