Is natural selection a fact or theory?
Natural Selection is a theory
The idea that organisms can evolve by micro and macro evolution is a fact. However there is no definite factor to what produces evolution.
Natural Selection is a theory because it is backed by observable evidence but is not considered the definite cause as to why organisms can evolve due to surrounding debate.
That natural selection occurs is a fact, that natural selection can cause changes that result in an increase in complexity required by Neo Darwinian Evolution is a theory.
Natural selection can be easily observed, in nature. Foxes that live on the edge of the snow line in the Northern Hemisphere are light grey. Foxes that live within the snow line have "adapted" and are mainly white. while foxes south of the snow line in the forest are mainly brown.
Another example is before the forest in England were covered in black residue from coal burning the moths were mainly white. During the industrial revolution while the forest were covered in black residue the moths were mainly black. When the burning of coal was refined and replaced by natural gas and oil the black residue in the forest disappeared and the moths reverted to being mainly white.
These example show that natural selection occurs. These examples do not show that natural selection can result in new forms that have more information and are more "advanced" than the previous forms.
Natural selection is a fact but is part of the theory of Neo Darwinian Evolution, which though unproven is widely accepted.