Texas Science Teacher Resource Hub

In everyday talk, "theory" can mean a hunch. In science, a theory is a well-tested explanation built on a large body of evidence. Scientific theories about the origin of the universe are supported by multiple independent lines of data, including galaxy redshift, the cosmic microwave background, and the measured abundance of hydrogen and helium. A theory becomes accepted because the evidence keeps lining up with it, and it can still be revised when new evidence comes in.

Scientists have a leading theory that describes the universe expanding from a hot, dense early state about 13.8 billion years ago, and the data strongly supports it. But what caused that early state, what (if anything) came before it, and the exact details of the very first moments are still active questions in physics. Theories about the origin of the universe have been refined many times as new data has come in, and they will keep being refined. That's how science works.

The data suggests something stranger. Distant galaxies in every direction show light shifted toward the red end of the spectrum, which means the space between us and them is stretching. Galaxies aren't moving through space like debris from a bomb. The space itself is getting larger between them. The balloon analogy helps here: dots drawn on an inflating balloon move apart because the surface stretches, not because the dots are flying through the balloon.

The evidence was collected piece by piece over almost a century. Hubble showed the redshift of distant galaxies in 1929. Penzias and Wilson detected the cosmic microwave background in 1964. Measurements of hydrogen and helium ratios were refined over decades. Each new piece of data either matched what existing theories predicted or pushed scientists to revise them. Students should see this as a long process of analyzing data, not a single "aha" moment.