、？
In this boundless universe, are we humans really the only ones?
Literal
This boundless universe [in-に] exist [nominalizer-の] [topic-は], we humanity only is [question-か]?
A philosophical/SF question. 果てしない ('boundless,' 'endless,' 'limitless') is an い-adjective built on 果て ('end, extremity') + ない — literally 'without an end.' Often used of vast things: 果てしない宇宙 (boundless universe), 果てしない道 (endless road), 果てしない夢 (endless dream). The structure ~のは~だけ is the cleft 'X is only Y' — isolating 'we humans' as the only occupant of the universe. The rhetorical force of the question is to invite doubt about that monopoly.