If we (the galaxy) were traveling close to the speed of light; relativity says we would need proportionally more energy to go faster. Given that relative to the cosmic microwave background, the Local Group is moving by 600 km/s; that's 0.2% of the speed of light. Is it possible to look at this in terms of mass/light speed.
i.e.light speed =infinite mass.
.2% light speed =x mass.
stationary = mass.
x = speed