I found the culprit, I experimented with a backup app, and when I abandoned the app, many, many GBs were not uninstalled. Thanks to all who responded!
I have only about 30 GB of user data, but the used space on C: is 155 GB. The last time I looked at 'Properties', it was much, much less than 100 GB. I don't know when the jump happened. What could cause this jump?
XPS 8700, used space on C: way too large