What you learned in school
Cosmology textbooks taught the Steady State theory as the leading scientific explanation for the universe. Students learned that the universe had always existed in roughly the same form - expanding but maintaining constant density as new matter was continuously created. This elegant theory, championed by prominent astronomers like Fred Hoyle, avoided the problematic concept of a "creation event" and seemed more scientifically respectable than theories requiring a beginning.
What we know now
Cosmology classes taught the Steady State theory, which proposed that the universe has always existed in roughly the same form. As it expands, new matter is continuously created to maintain constant density. This was presented as the leading scientific theory, supported by prominent astronomers like Fred Hoyle. The discovery of cosmic microwave background radiation in 1965 provided strong evidence for the Big Bang theory instead. The universe had a definite beginning and has been expanding and cooling ever since. The Steady State theory was largely abandoned by the scientific community.