I don’t recall the exact percentage, but historians found that something like 1/3 of the US mainland territory was never officially ceded from previous treaties (and something close to that likely ceded due to fraud, but that’s obviously more likely to resonate with moral argument than stand up in a court of law in the current legal system).

My city (Seattle) is undisputedly Duwamish territory. The way that’s been dealt with is to deny the Duwamish nation federal recognition of existing at all, so legally they have no claim as a nonexistent nation.