Scientists believe the Universe is infinitely large and does not end. However, only part of the Universe is visible from Earth. The Observable Universe is a sphere around the Earth, and anything beyond this sphere cannot be observed from Earth because of continuing expansion over a long period of time.
The Observable Universe is a sphere with the Earth in its center. It has a radius of 13.7 billion light years, which is the age of the universe. Things beyond this sphere are invisible because the Universe is expanding faster than the light can reach Earth. This expansion has been happening since the Big Bang and is causing objects to accelerate away from one another at an increasing rate.
Although from Earth it appears that other objects are moving away from the Earth, everything is in fact moving away from each other at equal speed as the Universe expands. This equal expansion is caused by the fact that the Big Bang did not create the Universe from a single point, but from everywhere at once. This expansion and finite speed of light means parts of the Universe never have the potential to be observable from Earth even though light is travelling towards Earth.