Why natural selection is not just a theory?

In science, the meaning of "theory" is different than the one we use commonly.

In the day-to-day, "theory" means a supposition or speculation, while a scientific theory is an explanation of a certain aspect of nature that is supported by evidence collected from several sources, and that has been tested repeatedly following the scientific method. A scientific theory is considered to be a truth, which is the case for natural selection and evolution.

regents of the university of california v. bakke, 438 u.s. 265 (1978), was a landmark decision by the supreme court of the united states. it upheld affirmative action, allowing race to be one of several factors in college admission policy.