Light aircraft pilot reported an NMAC with another aircraft in the vicinity of DMW airport.

Narrative

I was flying Aircraft X with an instructor. We had taken off out of DMW and worked our way northeast to the practice area. We were doing climbs and descents because we were practicing engine-out procedures. The incident happened during one of the climbs; and I think we were climbing through 2300 MSL when it happened. I am not sure about the exact location; altitude; or time because we continued the training after the incident. I was in the left seat. As we were climbing; I saw a white and green low wing aircraft disappear under the right side of the engine cowling. He was flying at roughly a right angle to us; right to left. There was no time to take evasive action. I looked out the left side of the aircraft (my side) and saw him pop out. He looked wings level but I couldn't tell if he was climbing or descending. I had the impression; though; that he was in level flight. When he popped out on my side; I estimate that he was about 200 ft below us and had passed about 200 ft in front of us. It looked 'slicker' than Aircraft Y and it reminded me of a Aircraft Z; but I think it was larger than that. I had the impression that he may have been going into DMW since his altitude was so low; but I don't see any arrivals to DMW in FlightAware in the appropriate timeframe.