Theory (noun, “THEER-ee”)
This is an explanation about the way the natural world works. A theory explains not just what happens, but also how it happens. Theories are based on experiments, observations and facts that many scientists have confirmed, over and over. Theories also help scientists make predictions and form new questions.
Sometimes, people will say they have a theory, but they actually have a hypothesis. A hypothesis is an idea that someone can test. A theory is much more than that. It’s been tested in many ways, by many people.
A theory also organizes knowledge that applies to many different cases. For example, evolution is a theory. It states that groups of organisms change over time, and explains how that change happens: some members of a species survive to pass on their traits and others don’t. We call this a theory for two reasons. First, a huge amount of data shows that species change over time. Data from fossils show this. Data from organisms alive today show this as well. Second, these and other data explain how species change. The theory applies to all living things — animals, plants, fungi and bacteria all evolve.
Theories aren’t forever, though. If there’s new evidence that doesn’t quite fit, theories can be updated. For example, the ancient Greeks believed that diseases could spread from “seeds” of illness. Some ancient Romans also thought that tiny creatures in swamps could make people sick. In the 1800s, Louis Pasteur and Robert Koch showed that tiny organisms did cause disease — bacteria and viruses. Now, the germ theory of disease includes bacteria, fungi, viruses and more.
In a sentence
Scientists have a theory that explains how the universe began — the Big Bang theory.