Thank you so much for your kind comment.
You correctly guessed that you have to get rid of the darkness by hitting them with the beams! Sadly, if the darkness eats your butterfly before you do, the game brings you back to the start.
Most of our players didn't realize this, which means it's a design flaw on our side, so apologies for that, we're working on improving clarity.
We hope the next update will improve your experience even further :)