The estimate that there are trillions of galaxies in the universe is based on observations and calculations made by astronomers and cosmologists. While it is not possible to count each individual galaxy, scientists use a variety of methods to estimate the number.
One of the early attempts to estimate the number of galaxies was made by the famous astronomer Edwin Hubble in the 1920s. Hubble observed that galaxies are moving away from each other and that the more distant a galaxy is, the faster it appears to be receding. This led to the realization that the universe is expanding. By studying the rate of expansion and the distribution of galaxies, scientists can make statistical inferences about the total number of galaxies.
In more recent times, astronomers have used advanced telescopes, such as the Hubble Space Telescope, and large-scale sky surveys, like the Sloan Digital Sky Survey, to observe and catalog galaxies. These observations provide data on the distribution, characteristics, and clustering of galaxies in different regions of the sky. By extrapolating this information to the entire observable universe, scientists can estimate the number of galaxies.
It is important to note that the estimate of trillions of galaxies is based on the observable universe, which is the portion of the universe we can currently see. The actual total number of galaxies in the entire universe, including beyond our observable horizon, remains unknown. The universe is vast and continues to be a subject of ongoing research and exploration.