According to current astronomical theories, the universe was born about 13.8 billion years ago and has been expanding since its birth. After the birth of the universe, a large number of elementary particles were formed, which constituted everything in the known universe except dark matter and dark energy. Because hydrogen contains only one proton and one electron, it is the most common element in the universe. Under the action of gravity, hydrogen forms the most basic unit in the universe - stars. These stars come together to form galaxies large and small. The Milky Way, for example, is a galaxy containing hundreds of billions of stars that is more than 100,000 light-years in diameter.
The part that humans can observe now is only about 93 billion light-years away, known as the observable universe. Scientists estimate that there are at least 23 trillion light-years left in the universe that cannot be observed.
While it's hard to have an exact answer to the number of galaxies in the universe, scientists still have a way to give a rough number. According to the part of the universe that humans can observe, the volume size of it can be determined, and the density of matter in the universe can also be measured.
According to data measured in 2015 by the Wilkinson Microwave Anisotropy Probe (WMAP), the mass and energy density in the universe is about 9.9× 10^-27 kg/m3, extremely empty. In this way, scientists can observe the total mass of matter in the universe. Scientists estimate that the total amount of matter in the observable universe is about 3.4×10^(54) kilograms, including ordinary matter, dark matter, and dark energy. Dark matter and dark energy account for 95% of the universe's total matter, and the remaining 5% is ordinary matter. Now, the galaxies in the universe that humans can directly observe are made up of stars formed by ordinary matter.
Galaxies are large and small, and their masses are average, so you can estimate the approximate number of galaxies in the observable universe. After calculations, astronomers determined that there are at least 2,000 billion galaxies in the universe. In addition, astronomical observations show that galaxies are evenly distributed throughout the universe. If we take a picture of a certain area of the sky with a telescope, and then count how many galaxies there are in the photo, and then according to the proportion of the area taken in the entire sky, we can also know how many galaxies there are in the universe.
Between 2003 and 2004, Hubble spent more than 100 days taking a small piece of the sky to synthesize images of the sky of about 10,000 galaxies – hubble ultra-deep field (HUDF). The farthest galaxy from Earth is about 13 billion light-years away. This small area accounts for only one-twelfth of the total area of the sky. In 2012, Hubble also photographed hubble's extremely deep field, which photographed about 3,200 of its space, including about 5,500 galaxies, some of which are even 13.35 billion light-years away from Earth.
Based on this, scientists speculate that the number of galaxies in the observable universe is about 100 billion to 200 billion. Due to limited observational capabilities, the data significantly underestimates the number of galaxies in the universe, or only 1/10 of the calculated results.
So since there are so many galaxies in the universe, will there be planets with life like Earth? The scientific community generally believes that extraterrestrial civilizations and life do exist, and they may not be as advanced in science and technology as human civilization. Or maybe there are civilizations that are more developed and advanced than human civilizations, but they may be too far from the earth to find them. After all, the Spiral Arm of Orion, where the solar system is located, is strictly speaking the outer reaches of the Milky Way, and the density of stars is much lower than that of the central region of the Milky Way.
NASA scientists believe that as long as the probe technology is in place in the future, it is only a matter of time before we find evidence of life or the existence of life on other planets in the solar system.
So with so many galaxies, how big is the largest galaxy? Astronomical observations show that the largest galaxy discovered so far is a galaxy called IC1101 in the Abel 2029 cluster about 1.05 billion light-years from Earth and 4 million light-years in diameter. The galaxy is 20 times larger than the Milky Way. Due to its enormous size, the galaxy, or Milky Way, has hundreds of times as many stars and contains more than 100 trillion stars.
Based on the huge size of IC1101, scientists speculate that the galaxy may have been formed by the collision and merger of multiple galaxies, otherwise it would not be so large. In fact, the Milky Way will merge with the Andromeda galaxy in billions of years, when the number of stars in the newly formed galaxy will reach more than 100 billion, but these will still not be enough to compete with IC 1101.
In addition to the largest galaxies, astronomers have discovered Segue2, the least massive galaxy in the known universe.
Discovered in 2009 by the Sloan Digital Sky Survey, the Segue2 galaxy is about 110,000 light-years from Earth and only 220 light-years in diameter. Satellite galaxies (or dwarf galaxies) located near the Milky Way. The interior is estimated to be about 1,000 stars. It is only 550,000 times more massive than the Sun, and is not as large as the central black holes of some large galaxies.
There may still be smaller galaxies in the universe, but due to the conditions of observation, and the light emitted by these galaxies is so weak that humans cannot detect them, it does not mean that they do not exist. In short, there are more galaxies in the universe than we can imagine. As Karl. Sagan once said, "There are more stars in the sky than sand on Earth." But based on what is known now, this sentence seems to be somewhat conservative.
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