I hate the use of 'we', as it is being used in the title, the question and the answer.

Put simply, 'we' cannot know anything. Knowledge only exists in the mind of an individual. No one is able to say what another knows. And unfortunately, most people don't even know the difference between knowing and believing!

At best, use of the term 'we know' is a shortcut to saying 'some scientists claim that'.

I can only know that claim to be true once I have verified it for myself. Until that time it can only be a belief or hypothesis.