Translation:We are talking about the Universe during the lesson.
I have never seen the word lesson used to signify the interval of time spent with a teacher and a group of pupils. It's either a class (school) or a lecture (university/college). Class can also pertain to adult education in professional or specialized environments outside of college. The only place I have ever seen "lesson" used is in books, or in curricula documentation, describing a unit of content, and thus awkward to juxtapose with notions of time, unless we are talking about indeterminate time, like in "During their lessons the teacher and his student developed a lasting friendship". Having read how a lecture at university is not "lekcja", I would suggest that the correct translation of "lekcja" is "class", not lesson.