US Dollar index is just a number that lets you track performance of the instruments comprising the index, so you can't invest in it directly. But you can buy US Dollar index futures or funds or invest in its components.
The lowest ever quote of US Dollar index is 89.130 USD. It was reached on Jan 6, 2021. See more data on the US Dollar index chart.
US Dollar index reached its highest quote on Sep 28, 2022 — 114.670 USD. See more data on the US Dollar index chart.