From the ancient primitive era of drinking blood and hair to the current technological era of flying into space, the real development of human technology only three hundred years.

But what we have achieved is amazing, people's quality of life has been greatly improved, and at the same time, eyes have been turned to the sky and the universe has been explored in all directions.

We often use the word "vastness" to describe the universe, but if we ask how vastness the universe is, there seems to be no more specific concept. For a long time in the past, the Earth's atmosphere has severely limited mankind's ability to observe the depths of the universe, and only by stepping outside the Earth can we truly appreciate the silence and vastness of space.

A series of photographs from a large optical space telescope gives us a glimpse of stars and galaxies shining in the depths of the universe. So another question arises: How many galaxies are there in the universe?

We don't see every corner of the universe, so how do we arrive at specific data?

To answer this question, the Hubble Telescope, which is already aging, is indispensable. In the past three decades of its work, it has accumulated more than one million effective observations, and it has given answers to the questions we ask today time and again, and continues to refresh people's knowledge.

In 1995, five years after the Hubble Space Telescope was launched, scientists pointed it at a region of the sky above the Big Dipper spoon where, to the naked eye on Earth, there was almost nothing. This small area is the size of a coin, or 26 millionths of an inch of the entire sky.

The Hubble telescope took a total of 342 shots of this nothingness over 10 days, covering ultraviolet, visible and infrared wavelengths. To get more light in, Hubble typically took 20-40 minutes for each shot, giving the researchers 342 images. After superimposing these photos, we see the first Hubble Deep Sky Field.

In this coin-sized square, there are at least 1,500 galaxies, large and small, each of them similar in size to the Milky Way. These galaxies are far from Earth and close to it, ranging from a million to ten million light-years away, and they are shown in bright blue and purple, while those that are far from Earth reach ten billion light-years away, and their wavelengths have been stretched to infrared wavelengths at great distances, so they appear faintly red in the diagram.

With this photo scientists estimate that there are at least 100 billion galaxies in the current observable universe, the first human estimate of the lower limit of galaxies in the universe, a number we already seem to have little idea of, yet this is just the beginning.

After the great success of the Hubble Deep Field, the Hubble Telescope made many more deep-sky observations in succession.

In 2003, the Hubble Telescope pointed its lens in the direction of the constellation Cepheus, an area located at 3h 32m 40.0s longitude and -27°47' 29" declination, which is only 3 square arc seconds in extent, and stretched its arm out towards space, which is the size of a fingernail cap at this time is the area of the Hubble Telescope for this shot, where no objects can be seen with the naked eye, which is far from the many stars of the Milky Way, and less interference.

In the second phase, Hubble's newly installed third-generation wide-angle camera and infrared camera were used to take 976,000-second exposures of 800 times, and the analysis showed that the number of galaxies in Hubble's super-deep field is at least 10,000, and most of them are 1-4 billion times fainter than the faintest objects visible to the human eye. The farthest galaxies from Earth are 13 billion light years away.

Based on the number of galaxies contained in this ultra-deep sky field, scientists have re-estimated the number of galaxies contained in the universe. Within the observable universe, the number of galaxies in the universe reaches at least 150 billion, so how did scientists estimate the number of galaxies in the entire universe from deep-field photographs?

Modern astronomy considers that the density of distribution of matter in the universe is generally relatively uniform. This means that by counting how many galaxies are in the photograph and calculating the proportion of the photographed area to the entire sky, we can roughly estimate how many galaxies are in the entire universe.

Initially, scientists estimated this value to be 176 billion

But then a team of researchers from the University of Nottingham simulated the universe in a computer, which was based on the Big Bang theory, adding various mathematical models and physical parameters, and then combined with the Hubble Telescope's observations, and finally calculated that within the current 93 billion observable universe, there are at least two trillion galaxies, which is as many as ten times the previous estimate.

In fact, due to the blockage of cosmic dust and the distance, many galaxies are not observed by us. And the above statistics are based on the current observable universe.

Beyond the observable universe, there are universes that we cannot see, where the light emitted has not yet reached Earth, and at the current rate of expansion of the universe, their light may never reach them, so the true size of the universe has long since far exceeded our perception.