The leading theory regarding the origin of the universe is the Big Bang Theory.
About 13.7 billion years ago everything that is present in the universe today is packed together in the space effective size of 0.
At that time not even atoms existed. It was the hot and dense soup of subatomic particles.
Since the beginning, the universe has expanded rapidly. Atoms have formed as it cooled and expanded, Stars were born.
Our Universe is like a time machine. If we see the distant Galaxies or any region of space here from the earth, we actually are seeing the past. Just because of light from the current event doesn’t reach the earth. So we are actually seeing the younger version of galaxies may be millions or billions of years old.
It is also important to note that there is no center of the universe. Because since the origin of the universe everything is expanding away from everything.
The universe has a filamentary structure. The gravity from galaxies is pulling them all together even though the expansion is pulling them apart.
The rate at which the universe is expanding since the origin of the big bang is determined by Hubble constant. Currently around 80,000 Km/h per million light-years. That is for a section of the universe that is across one million light-years it expands 80,000 km every hour. So the universe is expanding even faster than the speed of light that means light emitted from the distant galaxies will never reach us.
Universe just changed in size from very small to very large. So if we want to know where is the origin of the Universe? the answer is everywhere. It does not have specific co-ordinates because it is the expansion of the coordinate system itself.
After the big bang, the universe was hot around 2500 ºC. This heat may have emitted visible light. But the expansion stretched the wavelength of Electromagnetic waves by the factor of 1000. This means the photon can no longer arrive in the form of visible light but rather as Microwaves.
So just like seeing the distant galaxies millions of light-years away using visible light. We can see what the universe looks like before galaxies even existed by observing these microwaves.
So the Oldest thing that we can observe are Microwaves.
We cannot see beyond that as before that atom was just becoming a thing and light cannot travel through this hot and dense soup of fundamental particles.