Pope Pius XII saw the Big Bang theory as a type of scientific proof for the existence of God, and did not see it as contradicting the Catholic faith.
The Holy Father wholeheartedly embraced the theory, presented in 1920 by a Belgian priest, Msgr. Georges LeMaitre. Addressing the Pontifical Academy of Science in 1951, Pius XII gave a speech titled, "The Proofs for the Existence Of God in the Light of Modern Natural Science, saying:
True science discovers God in an ever-increasing degree — as though God were waiting behind every door opened by science. ... [It] perceives and recognizes the work of creative omnipotence, whose power, set in motion by the mighty Fiat pronounced billions of years ago by the Creating Spirit, spread out over the universe, calling into existence with a gesture of generous love matter bursting with energy.
The Fiat to which Pius XII referred is part of the creation narrative found in Genesis 1:3 that reads, "In the beginning, God created Heaven and earth. ... And God said: Be light made. And light was made."
This first instant of momentous creation of inorganic matter is supported, according to Pius XII, by the Big Bang theory. During his above-mentioned address, the pope spoke enthusiastically of the theory:
In fact, it would seem that present-day science, with one sweeping step back across millions of centuries, has succeeded in bearing witness to that primordial Fiat lux ["Let there be light"] uttered at the moment when, along with matter, there burst forth from nothing a sea of light and radiation, while the particles of chemical elements split and formed into millions of galaxies.
This first day or period of creation doesn't endorse a theory of evolution nor does it address the need for God to create life in one of the six periods of creation. It can be understood, however, as a way in which God brought inorganic creation into being.
Watch the panel discuss how natural truth never contradicts supernatural truth in The Download—Faith and Science.