The theory that the universe is expanding is based on SR and GR.
A more simple thought is that our tools and methods are improving allowing scientist to see more and further. First there was the solar system. Then the Milkyway galaxy. Then other galaxies were found. After that the galactic super clusters.
I am of the notion that our scientist are getting better at seeing great distances. When we send telescopes outside the solar system where photons are collected before hitting the solar winds and the heliosphere, we will have a far better picture of the universe. Also array telescopes seperated on an orbit between Earth and Mars will bring greater focus and reduce error.