The phrase "a plain fact" is correct and usable in written English.
It can be used to emphasize that something is straightforward and indisputable.
Example: "It is a plain fact that exercise contributes to better health."
Alternatives: "an obvious truth" or "a simple reality".
To them, inefficient markets are a plain fact.
It is a plain fact, not an excuse".
It's a plain fact that humans can marvel for only so long.
I'm brown, and that's a plain fact for anybody to see, she said.
"It is regrettable but a plain fact that child poverty is the biggest factor limiting children's potential.
But it is a plain fact that the act of collective worship is not altogether unproblematic in our schools today.
"You don't want to concede at any time whether you are a defender or a striker; the plain fact is we have conceded another three goals on our travels today because a clean sheet today and we could have won the game.
She notices her belated preference for black men as a matter of plain fact, explicable partly because a lover who is "white and well-bred" was a "status symbol" for some black men: "Deplorable although I can't help being grateful for it".
They argued that the former, because it's invented, is bogus and worthless; I countered that there's a difference between plain fact and timeless truth.
In this final chapter Mr. Raine's admiration for Eliot — which helped him write so eloquently about the poet's work in the book's earlier chapters — leads him into a state of numbed denial, afflicting him with an inability to recognize the plain fact that a great artist, one of the 20th century's pre-eminent poets and arguably its premier modernist, was also a terrible bigot.
Repeatedly, and quite often petulantly, the mayor intoned, "I am not a candidate" — a present-tense assertion of plain fact — while his lieutenants busily whispered to reporters all the things being done to test his appeal beyond the Hudson.
