Tell about Galaxies ?

Counting them seems like an impossible task. Sheer numbers is one problem

Another problem is the limitation of our instruments

To get the best view, a telescope needs to have a large aperture.

Perhaps the most resonant example of this fact is the Hubble eXtreme Deep Field (XDF), an image made by combining 10 years of photographs from the Hubble Space Telescope.

The telescope watched a small patch of sky in repeat visits for a total of 50 days

While estimates among different experts vary, an acceptable range is between 100 billion and 200 billion galaxies

When the James Webb Space Telescope launches in 2020, the observatory is expected to reveal even more information about early galaxies in the universe.

collected 10 days' worth of observations.

The result was an estimated 3,000 faint galaxies in a single frame, going as dim as 30th magnitude.

As the Hubble telescope received upgrades to its instruments, astronomers repeated the experiment twice.

In 2012, again using upgraded instruments

scientists used the telescope to look at a portion of the Ultra Deep Field.

Even in this narrower field of view, astronomers were able to detect about 5,500 galaxies. Researchers dubbed this the eXtreme Deep Field.

All in all, Hubble reveals an estimated 100 billion galaxies

universe or so, but this number is likely to increase to about 200 billion as telescope technology in space improves,

Whatever instrument is used, the method of estimating the number of galaxies is the same.

You take the portion of sky imaged by the telescope (in this case, Hubble).

Then — using the ratio of the sliver of sky to the entire universe — you can determine the number of galaxies in the universe.

This is assuming that there is no large cosmic variance, that the universe is homogenous,

"We have good reasons to suspect that is the case. That is the cosmological principle."

The principle dates back to Albert Einstein's theory of general relativity

Einstein said that gravity is a distortion of space and time.........

With that understanding in hand, several scientists (including Einstein) tried to understand how gravity affected the entire universe.

The simplest assumption to make is that if you viewed the contents of the universe with sufficiently poor vision,

"That is, the matter in the universe is homogeneous and isotropic when averaged over very large scales. This is called the cosmological principle."