Here's a good explanation: http://notjustatheory.com/
The author wrote this to rebuttle creationist arguments, but it's a good explanation for what any scientific theory is, not just evolution.
i was not crying in the shower
i just realized how i'll never get to meet darth vader and then soap got in my eye - Kylo Ren
People often think that a theory is something that hasn't been thoroughly proven and tested, when in many cases, it has. See: The theory of global warming, gravity, round earth, etc.
No theory can ever become a fact in science, but it can be thoroughly supported by research evidence.
"Trump took a dump." -CNN
"FAKE NEWS FAKE NEWS GLORIOUS LEADER DOESN'T POOP!"
Hypothesis is frequently presented as fact, until you argue a person down until to such a point that they innocently declare "Oh, sorry? This was just a hypothesis. It's not what I think, not at all". That's my hypothesis about hypotheses. You may wish to go for a more technical definition.
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If it is observable and verifiable it is a fact. What happened and how it happened so that you could observe it is theory (a tested and peer approved hypothesis about those events)
For example it is accepted as fact that modern humans first appeared around 200k years ago. However, evidence showing that it was actually earlier than that could possibly be found. It just hasn't despite the overwhelming amount of research done in this field. The more evidence fails to falsify it the more solid the fact becomes. This is how all science is structured. Everything can be falsified. Doesn't mean it will be but if the evidence is there it could be.
I realize that to the lay person facts that have the potential to be disproved must not be really facts. Science must be approached this way though so as to leave everything open to improving our understanding and modifying the way we think or approach about a given phenomena.
For example, what is supported as fact is that our bodies are comprised of trillions of individually functioning cells. The fact is that hypothesis not the cells themselves. Microscopes help us physically see the cells, the data or evidence, and confirm the hypothesis as fact.
We can then build on that conclusion and hypothesize that in order for all these individual cells to function as one body, or even a single organ, they would also have to somehow be communication with each other. Through the field of Biochemistry we can actually observe cells using chemicals to do this. The fact is that cells communicate with each other via chemicals not the chemicals themselves.
Does that make sense?
The Big Bang is a theory. The expansion is a fact.
So I don't really get what distinction Netherspark is making here either.