If nihilism was true - if there was no meaning in anything - then we could never know it to be true because we could never know anything.
Any evidence that nihilism was true, would refute nihilism - because if there is no meaning in reality then there can be no evidence.
What is peculiar is that people behave (and speak) as if there could be evidence in favour of nihilism - for example that the 1914-18 war or the Nazi Holocaust revealed that life was meaningless or whatever - but this is non-sense for the reasons above.
If there really was no meaning in existence - if it really was all random, contingent, purposeless - then we could never know this. We might suspect it, we might even believe it - but we could never know it and could never point to anything at all as evidence in favour of it.
Even one single piece of knowledge or evidence about anything at all would refute the idea that the universe had no meaning.
How, then, could so many people come to believe that the universe was meaningless and also to believe that they had strong grounds for believing that the universe was meaningless?
How could they believe this?
Yet this is the mainstream contention in the modern West.