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Moons orbit planets, planets orbit stars, and stars orbit the center of a galaxy.

So, my question is what does a galaxy orbit? The center of universe? (I know that the universe has no center)

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    $\begingroup$ Galaxies "orbit each other" and they are forming galaxy clusters $\endgroup$ – Gigi Butbaia Aug 4 '14 at 14:33
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    $\begingroup$ @GigiButbaia: that's just pushing the envelope: what do galaxy clusters orbit? :D. $\endgroup$ – Kyle Kanos Aug 4 '14 at 14:38
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    $\begingroup$ Galaxy Clusters orbit other clusters of galaxies :D (For example Virgo Supercluster) @KyleKanos $\endgroup$ – Gigi Butbaia Aug 4 '14 at 14:40
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    $\begingroup$ There is also gravitation anomaly in space called The Great Attractor, and "All" galaxies are moving towards it. Wikipedia says that it has mass tens of thousands times that of the Milky Way $\endgroup$ – Gigi Butbaia Aug 4 '14 at 14:43
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    $\begingroup$ I made a similar question a few days ago and got a great answer: physics.stackexchange.com/questions/123875/… $\endgroup$ – Renan Aug 4 '14 at 16:05
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The term "orbit" means that an object moves around a point in space on a certain path. Generally, this center point is an object - like your examples - or, in a binary system, a point in space called a barycenter, around which both bodies orbit. The barycenter is located at the system's center of mass. Commonly cited examples of orbiting objects are the ones you gave above - moons, planets, and stars. But all of these objects orbit a center point.

If the universe has no center point, what can galaxies orbit? Each other? In some cases, this is true. The Milky Way is orbited by some smaller "satellite" dwarf galaxies, such as the Large and Small Magellanic Clouds. Andromeda, a nearby spiral galaxy, also has these satellites. But do the Milky Way and Andromeda orbit anything? No.

The Milky Way and Andromeda are part of the Local Group, our local galaxy cluster. They are gravitationally bound to each other. There is a center of mass in the Local Group, just as there is in a stellar system. But in the Local Group, the Milky Way and Andromeda are actually coming towards each other. A collision, and eventual merger, between the two, is expected to happen billions of years in the future.

On a larger scale, the Local Group is part of the Virgo Supercluster. It contains many other galaxy clusters. It, too, has a center of mass. But all the galaxy clusters are not "orbiting" this center of mass. They are merely bound together, thanks to gravity.

On an even larger scale, "filaments" are created out of long lines of galaxy superclusters. In fact, at even larger scales, the universe is thought to be homogenous - essentially the same in all directions. There is no single point in space that is the center, like you said, or that is different rom all the others.

Of course, the universe is thought to be expanding. In that case, not only are galaxy clusters and superclusters not closely orbiting each other, but the space between them is causing them to move further apart!

Edit

Also, you might find good explanations and/or related questions on the Astronomy Stack Exchange.

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  • $\begingroup$ Re The term "orbit" means that an object moves around a point in space in a closed loop. That's true only in the non-relativistic two body problem. None of the objects in the solar system move in a closed loop thanks to perturbations by the other planets and general relativity. Yet we still can say that the planets orbit about the solar system. $\endgroup$ – David Hammen Aug 4 '14 at 15:42
  • $\begingroup$ Ah, true. I meant "closed loop" as an approximation, i.e. as opposed to an object wandering off into space, ejected from a system. But yes, you are right. Unstable orbits would also not satisfy the "closed loop" designation. $\endgroup$ – HDE 226868 Aug 4 '14 at 15:46
  • $\begingroup$ @HDE226868 Thanks for the answer. Was a good response. I'll take a look at the Astronomy SE. $\endgroup$ – Only a Curious Mind Aug 4 '14 at 16:07
  • $\begingroup$ The center of mass is essentially a barycenter. The Milky Way and Andromeda may have intersecting paths, but these paths do satisfy the definition of orbits around the barycenter. They may be highly eccentric orbits or pass through the barycenter, but those still constitute orbits. Gravity is a funny thing in that any objects bound to the same system by it can be considered in orbit of a common center. The orbits may be eccentric or degraded, but it is always considered an orbit so long as the objects are bound (not escaping) $\endgroup$ – Jim Aug 4 '14 at 17:15
  • $\begingroup$ I had understood "orbit" as used by LucasAbilidebob as a Keplerian orbit, typically an ellipse. LucasAbilidebob, had you meant that definition, or was I simply using a special Keplerian case? Interestingly enough, though Wikipedia does redirect "barycenter" to "center of mass", so my generalization could be wrong. $\endgroup$ – HDE 226868 Aug 4 '14 at 17:23
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Generalizing the term "orbit" to mean some larger object / collection of objects to which the object in question is gravitationally bound, I'd say that the Milky Way "orbits" the Local Group, which in turn "orbits" the Virgo Supercluster.

Beyond that, the expansion of the universe starts to dominate over gravitation. There are larger structures than superstructures, but it's a big stretch to say that the members of those very, very large structures are orbiting one another.

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