The Big Bang Theory
In the 1927, a French astronomer, Georges Lemaître, proposed the idea that in the distant past that the Universe started as just a single point in space, and as the Universe has been expanded as a great explosion to what is observable now. Two years later in 1929, Edwin Hubble reported that the most distant observable galaxies are moving away at a faster rate than ones closer to Earth. This observation, and much other evidence, now supports a Big Bang Theory.
The Big Bang Theory is a cosmological theory holding that the Observable Universe originated approximately 13.8 billion years ago from the violent explosion of a very small agglomeration of material of extremely high density and temperature (Figure 1.42). Current scientific though is that originally the material ejected from the Big Bang was too hot for subatomic particles with measurable mass to exist. It was probably many thousands of years after the Big Bang that it got cool enough for sub atomic particles and then atoms to form, and that gravitational attraction could influence the newly forming matter. Early in the history of the Universe matter began to condense and with time gravitation attraction pulled materials together to form galaxies.
In 2016, the Hubble Space Telescope was able to capture an image of the furthest distant galaxy known, estimated at about 13.4 billion light years away from Earth.
What is beyond the Observable Universe is unknown. See a NASA website about the Big Bang Theory.