UPDATE:  SOLVED by John at 12:12

Can you believe it?  May is already two weeks old.  I’m already looking forward to Autumn, my favorite time of year.  It’s about here that I start dreaming of cold, rainy days.  Do you remember the smell of burning leaves?  I associate that with Fall, although you never smell it anymore.  I guess it’s a sign of age, to look around and wonder, “what happened?”

Since I’m not getting any younger, I think I had better start today’s riddle… before I forget what it’s about!  You’re in reality today, but you know; reality can get pretty weird.

NASA/ESA Hubble Boomerang Nebula

You think of today’s answer as a “thing” of sorts, but it isn’t.  Not really.

It’s something of a concept.

It’s defined more by what isn’t there, than what is.

You could think of this as a destination, although it isn’t really a place.

States of Matter - old education model

We can see the goal, but we’ve never reached it.

This has an opposite.

We can’t get to that, either.

When you get really, REALLY close to this, something weird happens to matter.

We’ve been really, REALLY close.

This has made numerous appearances in SciFi, and can make for some graphically interesting special effects.

NASA/JPL Galileo - the lovely Europa

And there you have it.  Hopefully this will give you a moment’s thought.  I’m in the comments, as usual.

Awwwwwwww!