I suppose we will never know for absolute, positive, no other possibility that there wasn’t election fraud during the last election. There are many things like this, including every theory in science and every “belief” in every religion. That is just the way that the world works – and the nature of “proof.” Theories are presented, investigated, tested and considered from every point of view that we can think of. If we find evidence that the theory is wrong, if we test it and find it doesn’t “pass” the test, then we can definitely “prove” that the theory is false. However, we can never be certain that the next positive test isn’t followed up with a negative one, we can’t “prove” that every avenue has been investigated, that every test has been tested, or that we didn’t miss something important. As some point we finally say that while we can’t be certain that new information won’t turn up we accept the findings as “true” (which is different than proven). We think gravity works and will continue to work, we think aerodynamics will continue to keep airplanes in the sky, all of the things that have come from science and engineering are based upon “unproven” but “true” theories. So while it is not possible to “prove” that there was no significant fraud, enough investigations, observations, considerations and evidence has been found to show that it is “true”. It is true in the same sense that almost every election since the founding of the Country has been shown to be fair and the results have been accepted as being true.
That is not to say that there no major problems with our election system. There are. For example, the archaic electoral college system has opened the way to massive and pervasive disenfranchisement of millions of voters, almost all of which traditionally vote Democratic, by Gerrymandering districts in ways that award more electoral college votes than are supported by the population of the States. The Republicans have abused this to the point were it will heavily bias the outcome in favor of Republicans for decades into the future. It has gotten to the point where it is almost impossible to elect Democrats in many districts even when the popular vote would prevail for Democrats.
There are also often reports of highly illegal activities at the level of the precincts. For example, during the election between Gore and Bush, there were many reports of entire mail bags full of ballots from predominantly Democrat precincts being thrown into creeks and rivers, never being reported or counted. Did this actually happen? Probably. It probably happens during all Presidential elections. Is it enough of a problem to swing the outcome? Who knows? We have no evidence one way or the other.
Perhaps the worse problems that we have are associated with various successful efforts by the Republicans to disenfranchise poor voters through various means such voter ID laws, voter registration restrictions, unfair or illegal voter purges, felony disenfranchisement, and voter suppression by making it difficult to vote by the location or numbers of polling places. By far the worst of these types of voter restrictions apply to black and other minority populations, resulting in a massive under representation of votes that are historically cast for Democratic candidates. Some of this falls under the heading of “fraud”, but most is legal, but ethically and morally reprehensible.
We need to find a way to stop the impacts of Gerrymandering, makes sure that all people have ready access to polling places, eliminate the abuses of purges preventing millions of votes from being counted. We also need to find ways to prevent bags of votes being dumped, and we need a way for every citizen to have an easy way to verify that they vote was counted, and counted in accordance with the way that they marked their ballot. We need to find a way to get visibility into the entire voting process so that challenges such as Trump’s unsupported insistence in massive voter fraud is no longer possible. Basically, we need a complete overhaul into the voting system in the United States rather than continuing to rely on the current mix of outdated, archaic, fragile systems. If we don’t do this we will continue to have situations where the Supreme Court appoints the winner of the Presidential election, we will continue to have election after election where the Democrats win the popular vote but lose the election, where millions of eligible voters are prevented from voting, and where fraud remains a viable possibility.