Unless we stop to think about it, Earth appears to be standing still. We do not feel our true speed of motion around the Sun of about 67,000 miles per hour.
In turn, it is not hard to imagine the Sun also to be motionless. That is how it looks in textbooks. Yet the Sun plus Earth move together more or less randomly with respect to hundreds of other neighboring stars.
In turn, the Sun and another approximately 100 billion stars orbit the center of the Milky Way at a break-neck speed of roughly 550,000 miles per hour, given or take small perturbations to the motion caused by close interactions with other stars. The Milky Way, in turn, zips about the center of mass with respect to a few dozen other companion galaxies called the Local Group.
In the final rung of this ladder of motion, we measure the Local Group to move with a velocity of 1.5 million miles per hour in one particular well-defined direction. The question of the day is: what is it that is pulling the huge mass of the Local Group, which includes the Milky Way, towards it?
We certainly have part of the answer. By mapping out our local universe and measuring the velocities of the galaxies, we now know that the Local Group is drawn gravitationally toward a system of thousands of galaxies called the Shapley Supercluster. At the same time, the gravitational pull of this supercluster, mighty as it is, only accounts for at most one-half of the total speed of the Local Group, so what else is doing the pulling?
It would seem rather hard to imagine a gigantic structure of matter in space that is nearby and dominates the mass of the Local Group yet is unseen by astronomers. One way forward is to look not straight up into the sky, but rather through the Milky Way itself to the other side. This is a chalenge as our galaxy, like other spiral galaxies, is completely opaque to visible light.
We are able to peer through the dense clouds of dust and gas that are characteristic of our galaxy if we look at other colors, and in particular at radio wavelengths. By this approach there is finally some good news to report:
astronomers have indeed discovered another system of galaxies that can account for that other half of the gravitational attraction.
The initial results are encouraging, yet the confirmation is slow-going and requires further study. Through such studies we are seeing a bit further into the reasons behind the motives of the Milky Way's walk through space.