Please help me understand the concept of simultaneity. Is there such a thing as a universal "now," or is it strictly dependent upon specific relative locations agreeing upon a "now"?

Or maybe it's something else.

To put it into a hypothetical: I just sneezed on planet A. Suppose that intelligent beings similar to ourselves also possess the ability to sneeze on some other planet, planet B, in some other galaxy. Did I and one of those beings just sneeze simultaneously?

From what I gather, we may, if we are aware of each other's existence, calculate that the events occurred simultaneously, but someone on a third planet (C) in a third galaxy would correctly disagree, stating that I sneezed before the other. A being on a fourth planet (D) in a fourth galaxy would disagree again, stating that B sneezed before A. I still don't fully "get" relativity, but this much I can comprehend, all based on distance and relative positions. But correct me there if I'm wrong.

It's just that I have this gut feeling that there is a universal "now," that all sorts of events, perhaps an infinite number of "events" are occurring right "now" across the universe.

Is it just that we cannot ever know for certain which events occur simultaneously with the events occurring on earth, but that certain events elsewhere are indeed occurring simultaneously with earth?