Why do a lot of people think that the 2020 US Presidential Election was stolen from Trump, that Biden and the Democrats cheated, is there any proof?

Yes, i'm completely aware that the latest POTUS election is over, even though i didn't vote for him, i believe Trump won fair and square, but yet, the previous election, from 2020, that Biden won, a lot of people think that Biden and the Democrats cheated and stole that election from Trump, is there any evidence or proof?

Yeah in the entire history of US Presidential Elections, at least the Democrats have the class and decency to accept defeat, a loss, Democrats have never rioted or started an insurrection like the January 6th Capitol incident.

Anyway, do you think the 2020 US POTUS election was stolen from Trump? because i believe Trump still to this day, believes that Biden and the Democrats cheated in that election.

Why do a lot of people think that the 2020 US Presidential Election was stolen from Trump, that Biden and the Democrats cheated, is there any proof?
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