Short explanation:
For the majority of scientists world wide, which is a lot of scientists, to agree, the theory probably has really reliable sources and evidence that seems to be very solid for everyone.
Normally, when a group of scientists elaborate a thoery, there will always be another group of scientists that will study the same thing, and probably reach different solutions/conclusions, and therefore disagree. Since that wasn't the case in the example, the theory must be valid by a high percentage, if not 100%.
Important note:
There are many theories for natural selection, but we don't study all of them, only the most accepted ones, since they are considered to be more valid because of the amount of scientists that support them.
Hope it helped,
BioTeacher101