Thanks so much for writing to us here at TruthBook with your inquiry. It's always good to hear from our visitors and readers of The Urantia Book.
Before I even begin, I'd like to remind you of the disclaimer in the pages of the book regarding scientific discovery and instances of conflict between the book's cosmology and modern scientific progress:
That section is called Limitations of Revelation (Paper 101), and starts out this way:
"Because your world is generally ignorant of origins, even of physical origins, it has appeared to be wise from time to time to provide instruction in cosmology. And always has this made trouble for the future. The laws of revelation hamper us greatly by their proscription of the impartation of unearned or premature knowledge. Any cosmology presented as a part of revealed religion is destined to be outgrown in a very short time. Accordingly, future students of such a revelation are tempted to discard any element of genuine religious truth it may contain because they discover errors on the face of the associated cosmologies therein presented.
Having said that, we can go on to explore the answer to your question using the teachings of The Urantia Book, even though we now know that the descriptions in there may not harmonize with what our 21st century science is revealing to us.
Although it is not strictly a definition, the book says this about a superuniverse:
0:0.5 Orvonton is one of the seven evolutionary superuniverses of time and space which circle the never-beginning, never-ending creation of divine perfection—the central universe of Havona.
Click to read about the Seven Superuniveses
Each superuniverse contains 100,000 local universes, such as Nebadon, where we are, as well as 1,000,000,000 local systems.
Galaxies: Wikipeia defines a galaxy as "a gravitationally bound system of stars, stellar remnants, interstellar gas and dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way."
The galaxy we are most familiar with is, of course, the Milky Way galaxy, where our solar system resides.
15:4.8 The Milky Way galaxy is composed of vast numbers of former spiral and other nebulae, and many still retain their original configuration.
The discovery that you mention about the billions of galaxies could be a true observation, but science does not know what they are seeing just yet; it could be that they are peering into a neighboring superuniverse, for all we (or they) know...or, what they are calling "island universes" may be the one billion local systems, such as the Satania system. I don't know...
Nebulae: Again using Wikipedia, we learn that a nebula "...(Latin for "cloud"; pl. nebulae, nebulæ, or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others.
Here's an instance where modern science differs from UB teachings; The Urantia Book refers to Andromeda as a nebula.:
15:4.7 Andromeda, which is outside the inhabited superuniverse, is very active. This far-distant nebula is visible to the naked eye, and when you view it, pause to consider that the light you behold left those distant suns almost one million years ago.
Click to read that section on Nebulae - The Ancestors of Universes
The UB cosmology is unique in that it presents a picture of the cosmos that is so vast in its scope, that what modern astronomers think they see may eventually be better understood when they have discovered this incredible vastness and structure. Right now, modern astronomy is not even talking about superuniverses (there's no Wikipedia entry about it!), nor do they as yet understand the basic structure of the Master Universe with its seven superuniverses surrounding the Central Universe of Havona, as do Urantia Book students. Once they have figured all of that out, their discoveries may be better explained in context.
The cosmology presented in The Urantia Book is far in excess of what anyone knew before it was presented to us; nonetheless, with the advances of modern space exploration, we are bound to find more instances where some of these advances negate or modify the things that the revelators told us. After all, the book was delivered in the early 20th century and finished in 1934 - long before space exploration was really even seriously entertained. Here in the 21st century, we are in the nearly-constant process of updating even things that we thought we knew only a few years ago, not to mention the 60+ year-old revelations of TUB.
The revelators explain clearly in the disclaimer above that to give us too much information would have stifled our spirit of discovery; the rapid pace of exploration in the outer reaches of space is exciting for all of us, and I am sure we would not have wanted it any other way. There's a certain sense of satisfaction that comes from discovering things on one's own, I am sure you'd agree - especially if you're a scientist. And there may be other future discoveries that cause us to recognize that Urantia Book cosmology is incomplete; on the other hand, it may be just the opposite.
I hope this does not give you, or anyone else pause or doubt about the rest of the book. We are told clearly:
"Any cosmology presented as a part of revealed religion is destined to be outgrown in a very short time. Accordingly, future students of such a revelation are tempted to discard any element of genuine religious truth it may contain because they discover errors on the face of the associated cosmologies therein presented."
In other words, don't worry too much about these differences in cosmology; above all, don't throw the baby out with the bathwater. The religious and spiritual revelation of The Urantia Book that remains after the cosmological revelations have been sifted and put to rest is worth far more, in my opinion, than any revelation of a material nature. And that spiritual revelation can only be corroborated through the rugged proof of personal spiritual experience with God.
I hope you will agree.