[proaralyst]

US treasuries (bonds, notes and bills) are mostly considered as good as cash. They're used heavily in the international banking system (for payments, and as loan collateral themselves). They're also held to maturity by large institutional investors with predictable liabilities (ie payments) like pension and insurance funds. Banks will hold some of their assets (eg deposits) in treasuries too.

Also foreign governments and central banks will have holdings. Tether (the cryptocoin) is largely backed by US treasuries.