GA pilot reported a NMAC with a military helicopter that flew below them near mountainous terrain at night. Pilot took evasive action to avoid a possible collision.

Narrative

I was flying a Cessna 172S VFR solo about 12-13 miles west of the ZZZ airport. It was at night; skies were clear; and visibility was 10+ miles. I had a Sentry on board and was using it on my phone to help me detect traffic. I was coming from the ZZZ1 airport direct back to ZZZ (28nm); so because of the short distance I did not utilize flight following. At this point in the flight; I was about to make my initial call to ZZZ Tower. I did frequent scans to ensure no one was around me; and about 12 miles from the airport; I see a red flash on my phone with an aircraft; Aircraft Y; -300 ft from me. Immediately; I try to climb and turn away from it; however the aircraft was going ~240 kts over the ground; and I was only going about 95. At this point I was only at 2600 feet descending to 2500. The aircraft did not show up on my radar until it was immediately underneath me; I confirmed this claim with my friend; flying in the C152 behind me. He did not see the aircraft on his radar until it was extremely close to me. Later; I played back the flight; and I determined the aircraft to be a military Osprey coming from ZZZ2. Earlier; the track indicated there were two Ospreys; however one popped off of radar early on in their flight. Both of these aircraft were flying extremely low; extremely fast; and at night next to a mountain ridge. If they were on a military helicopter route; I was unaware. This occurred on Day 0 between XA30Z and XA44Z.