General aviation instructor pilot reported a near miss with two other aircraft performing aerobatics; while practicing maneuvers during a training flight. The maneuvered out of the area after the encounter.

Narrative

On Day 0 I was acting as CFI with a student of mine in Aircraft X. My student (private pilot; current) was piloting the plane and taking the left seat. We were practicing maneuvers in the area to the NW of ZZZ. At XB:40 local time at about 1000 ft. AGL; out of nowhere; we see two aerobatic airplanes in formation flight descending top to bottom right in front of us in what appeared to be the descend leg of a hammerhead. They were really close; so as to fill more than half of the windshield. We didn't see this coming despite having made several turns and clearing turns during our practice (as our ADSB track demonstrates). We also didn't pick them up on ADSB and the G1000 in our airplane didn't trigger a traffic alert (the airplane is equipped with ADSB-in). This event happened in a fraction of a second and all we could do was to move away from the area as fast as we could. Importantly; I suspect the two airplanes had their transponder off because after the near miss and while leaving the area; I saw them leveling off and slowing down low to the ground; at which point I checked again and again on the G1000; which still didn't report them as traffic.This is the second time I run into a close encounter with those two airplanes. The first time was a couple of weeks before when I was flying in another airplane in the area between ZZZ and ZZZ1. I saw those two aerobatic airplanes performing loops and other aerobatic maneuvers in tight formation from altitudes up to 3000-4000 ft. all the way to perhaps 1000 ft. and lower. During this first encounter I also didn't see the airplanes on my ADSB-in traffic page; hence I suspect those pilots turn off their transponder when performing such maneuvers. I called ATC asking if they were aware of any airplanes performing aerobatics in the area; and they told me they weren't.The airplanes that I saw were low wing. They appeared to be very maneuverable. I've done some investigation with my CFI friends and I discovered that I'm not alone in having had unwanted close encounters with those airplanes.