In short, we can't be certain about anything.

Due to our relatively small window of reference from a basically static position and inability to move throughout deep space, everything we know is based in what we know about how things interact from long distance observation. It is possible, however unlikely, that there are different means and changes to what we know as constants given circumstances that we cannot recreate here or Earth, or even that may change depending on distance throughout space that reaches beyond what we can observe. That is kind of what relativity is about, that we cannot say for certain everything is what we believe it is from a different point of reference. That is why we continue to explore what we can, and learn as much as we can with what we have, in hopes that some day more answers will come, but with them more questions will arise. Discovery is knowing that we can find those answers.