That’s a tough question because scientists like to know everything and we wish everyone felt the same as us, but I think the honest answer to this is: how uncertain we are about our results. The reason for this is probably not what you’re thinking, though.
A lot of people who distrust science are going to answer “Aha, I knew it! Showboating huckster scientists, you can’t trust them! They pretend to have all the answers and they know nothing!”
Well, that’s not true either. But let’s talk about uncertainty in the sciences for a moment.
Uncertainty is a fact of life. Only politics and religion deal with things in “certain” terms. In science, you can never be 100% certain of anything. You can be almost infinitely close to 100% certainty, but there will always remain the possibility, despite a mountain of evidence in your theory’s favor, that tomorrow someone will come up with an experiment you’ve never thought of that will somehow disprove everything you were so sure was true. Think that doesn’t happen? Go ahead and ask nineteenth-century physicists how certain they were that Newtonian physics held all the answers they would ever need. In fact, a lot of people at that time considered the science of physics to be nearing some sort of an end game since nearly all the answers had already been found. Oh my, how incredibly and unforeseeably wrong they were!
But that’s the wonderful thing about science. In religion, when a contrary idea arises, it is suppressed and often violently. Wars are fought over these things. In science, we investigate that new idea and if we find it works better than the old, we throw out the old or modify and adapt it as needed.
Scientists discuss uncertainty amongst ourselves all the time. We talk about the certainty of our results, the margin of error in our measurements, and along with our conclusions, we include alternative explanations for the observations we made. We actively acknowledge and allow room for our interpretations to be wrong, openly discussing the other possibilities, even when the evidence makes it obvious that we’re highly likely to be right. No scientist, speaking to any other scientist, would claim to be 100% certain about anything.
Lay people who are not intimately familiar with science, however, are used to dealing with absolutes. Are you sure X is true? Yes. How sure? Oh, totally sure. 100%! Is the Bible/Koran/religious-text-of-your-choice true? Yes. How certain are you? Oh, 100% certain. At least 100% certain! Probably 300%!
People often equate anything less than 100% certainty with not knowing, which is technically true I suppose, but not the point. Their pastor/priest/imam is 100% sure of what they “know,” their politicians are 100% sure of what they “know,” but only 96% of scientists agree that climate change is happening? And of those 96%, they’re only- say- 95% sure? Well, they just don’t know at all, do they? Physicists claim to know what happened during the Big Bang up until it was 1/10000000000000000000000000000000000000th of a second old (yes, that’s the actual number). What happened before that? We don’t know. And how certain are you of everything that led to that? Only about, say, 85% sure? So you really don’t know at all, do you? We’re only 99.x% certain that vaccines don’t cause autism? That’s not enough, I’m not getting my kids vaccinated!
I’ve had people ask me how certain I was, as a geologist, that the Earth is more than 6,000 years old. I’ve answered that I was 99.99% sure. I was being generous, allowing a 0.01% chance that some new scientifically testable/demonstrable explanation for… everything… might arise next week that totally turns the entirety of science on its head. Still, I was chastised and reprimanded for not seriously considering the 0.01% alternative… as though we ignore the alternatives in science.
This utter dismissal of anything that’s not 100% certain by most of the general public leads to us scientists being reticent of sharing our uncertainty with anyone outside of scientific circles. We don’t actively hide it- that would be extremely dishonest and shady. But we don’t usually advertise it to the general public either. We would much rather just share the results and conclusions of our research, making the methodology available to anyone interested in following it up.
Nothing in science (or reality, for that matter) comes with 100% certainty, but most people demand that certainty to accept anything. So while we scientists never claim to be 100% certain of anything, we also don’t tend to advertise the, say, 15% uncertainty in the results of our study outside of the journal we published it in because 90% of the public will dismissively brush it off as another case of a scientist not really knowing.