A theory is an explanation of why something happens

A theory is an explanation of why something happens. A Hypothesis is a guess or prediction about what will happen usually based on a theory.

The Periodic Theory explained that would repeat their properties in a periodic manner. The theory made predictions or hypothesis that certain new elements would be discovered, with specific properties of mass, protonic charges, and chemical reactions.

The predicted elements were discovered with the properties that were hypothesized by the theory. Because of this the theory was accepted as being true.

Einstein had a theory that mass bent space and this is what causes gravity. Based on his theory a hypothesis was made that light would be bent as it passed near a large mass like the sun. The prediction came true. This gave credence to his theory.