Scientists use a standard procedure for expanding our knowledge and understanding of the world, called the scientific method.
They identify an observation or process that they want to be able to explain.
They make a hypothesis, an idea which might explain the observation or process, and which can be tested.
They carry out experiments to test the hypothesis, which may support or disprove the hypothesis.
If the evidence supports the hypothesis, scientists will repeat the process many times with different observations and different experiments - perhaps using different instruments and different researchers.
If the hypothesis is tested in many different ways and is not disproved, it develops into a theory. A theory that is accepted by scientists may later be replaced by another theory if it is disproved.
This procedure, using experiment and observation to generate evidence to support or disprove a theory, is called empirical, meaning 'based on observation or experiment'.
Theories developed and supported in this way don't necessarily stay accepted forever. If new observations are made or new data comes along which disprove the theory, the empirical process must start again with a new hypothesis.