B737 Captain reported encountering severe turbulence resulted in momentary loss of altitude aircraft control.

Narrative

After a normal departure out of ZZZZ as PF; we got weather deviations from ZZZZ Center ATC to pick our way around some radar returns. It was night time and hard to see clouds so we mostly relied on our on board radar and ATC. Shortly after passing through FL180; and with the weather temporarily improving; I transferred controls to the CA so I could take my rest seat in the back and switch my seat with the Relief Pilot. After this exchange; I took my rest seat in the cabin. When what seemed to be 15 more minutes had passed; I noticed that we were encountering some considerable turbulence. I wouldn't describe it as violent; but there were several seconds of being very light in the seat followed by heavy in the seat; like a roller coaster. This repeated multiple times for maybe one minute or slightly less. The passengers in the forward cabin did not seem too alarmed and were mainly focused on keeping their drinks from spilling. Appropriate PAs were made. After my rest period ended and I retook my flying seat; my CA debriefed me on the turbulence and classified it as severe due to losing 600 feet of altitude and having difficulty controlling the aircraft at times. He stated no speed limitations were exceeded and that it was just past the radar returns. In ZZZ1; maintenance signed us off after gathering information performing an inspection.