Nothing in particular

Whoa! It's a bummer that the hidden quest isn't showing up. The official conditions haven't been revealed, but reaching certain levels or completing missions often holds the key. First, aim for level 41 and try to grow your Black Hole significantly, whoa!

It can differ from person to person! I installed the same app as my husband before, but he got a hidden quest while I didn't, even though he told me there was a hidden one when I installed it. When I contacted support, they said it can vary by individual. If you install the app after seeing information about it, the probability of getting a hidden quest is very high, but I think it's best to assume that it might rarely not appear. For stage clear type quests, they usually appear within 10 stages, so that might be a good reference point.

I see! Thank you!

It's just as written. Even if a hidden quest appears, the content and rewards might be different. There were times when the reward was coins for one person and gems for another.