We estimate that there are hundreds of billions of galaxies in the universe,but its only until you see that Hubble Deep Field, and you understand you are seeing a tiny little patch of sky and yet it is filled to the brim with galaxies.And that's telling you that this universe is an astonishing and crowded and wonderful place.The entire history of growth and change in galaxies is written in this single image of the Hubble Deep Field.What may be Hubble's most profound discovery cannot be seen at all, by any means.Something with so much power, it holds the entire universe together.For a number of years, astronomers like Kim Weaver have been surprised by the way galaxies behave.Our observations of things we could see, like galaxies, weren't making a lot of sense to us.For example, how a galaxy rotates.The arms of spiral galaxies seem to move faster than the laws of physics would allow.Astronomers spotted other oddities as well.
