Tether is by far the largest counterfeiter of USD and has already been shown to be backed largely by nothing.
The introduction of Tether coincided with the hijacking of the Bitcoin GitHub repo by the for profit company Blockstream.
Blockstream's Samson Mow had close ties with Tether and El Salvador dictatorial President who made Bitcoin a national currency on exchange for billions of dollars of free Tether dollars.
Tether is not and does not need to be backed by anything other than US intelligence agencies. There is no proof of this other than the fact that the larger USD counterfeiter is allowed to exist and control the price of the hijacked Bitcoin chain with the ticker, BTC.