The result of election was very unbelievable, Trump had have been counted ahead in a days in some Swing states, but after one night, the result was downturn. The duration of poll counting was unreasonable long.
Many people though it was a electoral fraud !
Trump's lawyers brought claims of election fraud to court, filing about sixty cases in various States. In most of these cases they made claims of fraud, but produced no evidence and witnesses proved unreliable and the cases were thrown out by the judges who were often appointed by fellow Republicans. Note that evidence was not presented, because if an attorney knowingly presents false information in court, that can be a crime in of itself. Officials, often Republicans who supported Trump in the election, investigated and found little or no fraud in their States' elections. Trump's Attorney General, his appointee, ordered an investigation of how states conducted the election, and AG Bob Barr reported that no significant fraud that could have changed the result had been found. Few errors in vote counting were discovered.
Swing States by definition are States where the vote is expected to be close and where either major party candidate can win. In 2016 Hillary was predicted by polling to win swing states that she lost. She complained about Russian interference and the Russia Collusion investigations began, but the fact is she simply lost.