The U.S. Air Force has assigned ten T-38C jet trainers to an F-22 unit to serve as training aircraft. Since F-22s don't expect to face anything as capable as an F-22 in the near future, anything that can move fast and maneuver like a jet fighter will do as an "opponent" aircraft. The T-38 is very similar, in shape and performance, to the American F-5 (a contemporary of the MiG-21), and does the adversary work for a fraction of the cost of using another F-22. Moreover, the T-38 is physically small, as are many likely F-22 opponents. Since the F-22 prefers to operate without using its radar, a small adversary is good training. Such an aircraft has to be detected using heat sensors, or, in daylight, the pilot's eyes.
The T-38 has been in use as a trainer for 47 years, and the U.S. Air Force has over 400 of them for pilot instruction. Air force T-38s are undergoing structural and electronics upgrades, to keep them going for another decade. Production of some 1,200 T-38s ended in 1972, so all existing models are over 39 years old. The five ton aircraft is actually a variant of the F-5 fighter aircraft (a few of these are still in service). Several other nations use the T-38, but many newer trainer designs are beginning to be bought as replacements.