Two modes of explanation dominate modern science. Necessity explains an event by appeal to a law.
Chance is invoked to explain random events.
Design , a third mode of explanation, is often dismissed as unscientific. This was not always so. Chicken eggs always produce chickens and go through predictable stages of maturation. However, modern science rejected Aristotle's metaphysics, and a mechanist paradigm became dominant. Causation was reduced to matter in motion, governed by necessity and chance alone.