The universe has around 100 billion galaxies, each with between 10 million and 1 trillion stars. Galaxies are organized into superclusters and filaments, with the Eridanus Supervoid being the largest void. The Milky Way has 200-400 billion stars and is part of the Local Group, which contains over 35 galaxies. The possibility of spacefaring extraterrestrial civilizations emerging in the Milky Way is low.
Along with the stars, the galaxy is the most important organizational unit in the entire universe. There are about 100 billion galaxies in the visible universe, each with between about ten million and one trillion stars. The average galaxy comprises about 1071 atoms and ten times as much invisible mass in the form of dark matter. Galaxies themselves tend to be arranged in large structures called superclusters, which in turn are arranged in massive filaments separated by immense voids. The largest of these voids, the Eridanus Supervoid, is nearly 1 billion light-years across, about 10,000 times wider than the Milky Way.
Our galaxy, the Milky Way, has between 200 and 400 billion stars, many of which are very faint from having run out of their nuclear fuel a long time ago. To get a dimension to its scale, if 100 billion humans could occupy the space around each star, then the entire Galaxy could provide space for about 1022 humans, more than the number of grains of sand on all of Earth’s beaches. The galaxy is only about 100,000 light-years wide and 1,000 light-years thick.
This means that the entire galaxy could be colonized in just one million years, about four times the lifetime of the human species, using spacecraft traveling at 1% of the speed of light. Most of the stars visible in the galaxy are main sequence stars, similar to our Sun. A minority are the Red Giants, stars much larger than our Sun but very widespread.
Although our galaxy is large – 100,000 light-years in diameter – it is small enough that if expansion-oriented space civilizations had emerged in the first 99.9% or so of its history, they would be here by now. This is a strong argument for the absence of spacefaring extraterrestrial civilizations in our Galaxy. It is possible that such civilizations have emerged only in extremely recent times, but the a priori possibility of such a recent emergence is rather low, of the order of 1 in 10,000 or 1 in 100,000, if the probability of occurrence is equally distributed over a large part of The existence of the galaxy.
The Milky Way is not a loner, but is actually a binary galactic system, gravitationally locked to our sister galaxy, the Andromeda Galaxy, about 2 million light-years away. Both our Galaxy and Andromeda are members of the Local Group, a collection of over 35 galaxies. The Milky Way is thought to be the most massive, though possibly the second largest, after Andromeda. The Local Group itself is embedded in the larger Virgo Supercluster, which contains over 100 galaxies in an area of 110 million light years.
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