In the 1970s astronomers discovered that something is pulling our solar system towards it. In the 1980s they found that it was actually pulling all of the surrounding galaxies. We have no idea what it is, but have aptly named it the Great Attractor.
The Moon orbits around the Earth, which in turn orbits around the Sun. Our Sun sits in the Orion Arm of the Milky Way, orbiting around an enormous black hole with as much mass 4,300,000 Suns. The Milky Way is part of a collection of about 30 other nearby galaxies called the Local Group which is around 10 million light years in diameter. The Local Group is a part of a group of 1,300 galaxies called the Virgo Cluster, which is itself a component of the Local Supercluster. This supercluster is about 110 million light years across and containing as much mass as 1,000,000,000,000,000 Suns, which is pretty impossible to visualize. Needless to say, it's big. A disconcerting discovery astronomers made in the 1980s led us to believe that there is some object out in deep space that is sucking in the Local Supercluster, and we can't even tell what it is. But we've imaginatively named it the Great Attractor.
The Great Attractor Discovery
One of the major discoveries of the space age was the microwave background radiation. If you point a microwave telescope at the sky, you will see light coming at you from all directions, which can be observed as static on a radio or TV. In the 1970s, precise measurements showed us that microwave radiation from one direction was a slightly higher wavelength than the opposite direction. This was a result of our solar system whizzing through space at an incredible speed, 600 km/s, and the background radiation getting slightly red shifted.
In 1986 a group of astronomers nerdily dubbed the "Seven Samurai" were creating a map of extremely distant regions of the observable universe. They found that galaxies were not spread evenly in space, but tended to form relatively small clusters with enormous expanses in between them. It was one of these expanses that the Milky Way and its surrounding galaxies was being pulled towards. Unfortunately, the direction of the Milky Way's movement was right on its plane, so that when we look towards the Great Attractor our view is obscured by all the stars in our own galaxy. Most objects that fall behind this Zone of Avoidance are hidden from us, making direct observations of the Great Attractor difficult. What our seven nerds were able to find out was the the Great Attractor is about 250 million light years away in the Centaurus Constellation and contains 10 times the mass of the Local Supercluster. It could be its own supercluster, or a dark matter object, or even some new classification of object.
The Not So Great Attractor
The Zone of Avoidance makes it hard to see the Great Attractor, but not impossible. Using an X-ray telescope, astronomers can see through our own galaxy and make observations behind it. That is exactly what happened in 2005, using the ROSAT telescope. When we looked at the Centaurus Constellation towards the Great Attractor, we found out that it's not quite as big as we thought it was. Another 250 million light years past the Great Attractor sits the Shapley Supercluster. This enormous collection of galaxies is the single biggest structure we have observed so far in the universe, and would account for a large amount of our observed movement. In fact, the Shapley Supercluster accounts for 66% of the Milky Way's movement through space. So once again, scientists have ruined a perfectly good mystery about the universe.
Great Attractor http://cow.physics.wisc.edu/~ogelman/guide/gr8a/greatA.gif