As you head out into space, you are heading back in time. Since there were no stars before 13.8 billion years ago, this means there is a sphere around us beyond which there is nothing to see. The wonderful thing is that we are back with the model of the universe proposed by the ancient Greeks. There is a huge sphere with the Earth at its centre, and photons from beyond that sphere have not had time to reach us yet. That sphere is getting larger as time passes, and the question of how much space is contained inside this expanding horizon will turn out to have an unexpected answer.
