UPDATE:  Solved by Kristian at 12:49 CDT

Welcome!   I see you made it back for another adventure in riddling.

We’re still in reality, but “reality” can encompass a lot of territory.  So, whenever you’re ready, jump right on in to the deep end of reality:

This is a modern discovery.

It would not have been known to our ancestors.

There’s something unusual about today’s answer.

In fact, it defines one whole side of normal.

Nobody is exactly sure why, either.

You could say this child is from a two-parent family.

We don’t know if there are any other sibs in the group.

We do know that every 2.5 days, this does something noteworthy.

Alright, chew on that for a while.  You know where to find me when you get an answer, but don’t delay.  I doubt this riddle stays open for very long.