A black cave deep under the earth in minecraft?

I'm new to Minecraft, and I decided to go on Creative and dig as deeply as I could. So I chose a spot near a mountain and started digging. It wasn't very long until I found myself in a deep underground cavern filled with what appeared to be glowing eyes or lights. I flew out of there as fast as my space bar would let me, but I still don't know what it is. Can anyone help me? Thank you.

Added (1). Just so you know, I'm terrified of the dark, both online and in real life. Any tactics for a scardey-cat trying to be brave enough to enter wherever this is on Creative?

If you're on creative, you can't die; don't be scared. You found a mine/dungeon is all.

Were they purple? It's quite possible it is an Enderman. When in Creative Mode you can't die or get injured by monsters, they don't even go for you while in creative mode so you have no need to avoid them.