PA-44 Flight Instructor with student reported a near mid-air collision with another aircraft during cruise.

Narrative

My student and I were climbing out of ZZZ towards the lake area as normal to practice maneuvers such as VMC demonstrations; engine shutdown; malfunctions; emergency descents etc.As we leveled out at 7500MSL all was normal. There was a haze layer that was around our altitude that made other aircraft quite hard to see with the reflection of the sun even if they had ADS-B which provided better situational awareness.I myself was scanning for traffic and also monitoring the student to ensure the safety of the flight.I took a glance down at my EFB to check for traffic on ADS-B and out the window for about 5 to 10 seconds and then back up to check on the student while also scanning for traffic.(Still saw nothing out the window and nothing on ADS-B)Next there is what appeared to be a Cardinal 177RG that came from the leftThe aircraft went directly under mine just barely missing each other by about 10-20 feet.After I had realized the situation and looked back at the aircraft they started descending.This aircraft was not on ADS-B and was not making radio calls.Afterwards I checked with another instructor on the radio in our practice area to see if my ADS-B out was working and they stated 'all good'.At this point I decided to discontinue the flight with my student and head back to ZZZ.