Lighting, Light Fixtures, Ceiling and Exhaust Fans - Help w/ new ceiling light ixture

Hello,

I am replacing my bedroom light fixture that has a dimmer switch. The junction box has 3 sets of wires. Each set has 1 black and 1 white. The previous fixture had 2 whites from the box wired together to the white fixture wire and a black from one of those sets of whites, that were wired together, connected to the black from the fixture. The other remaining wires were all twisted together and tucked up into the box.

Here is my problem. I seperated them back to their black and white pairs. I capped one off and tucked it in the box. The new fixture had 2 sets of black and white wires. I connected each black and white set from the box to the fixture sets. I connected black to black and white to white. When I turned on the breaker, it tripped. What did I do wrong? Thank you for your help!

Welcome to the forums! You didn't take a picture of the original wiring and relied on your memory to reconnect the wires.
In your box you have a cable (black/white) that is hot. You also have one that feeds a receptacle (probably) that should remain hot. The other cable is actually a switch loop, and it is not black/white, but black/black. Is there a piece of black tape wrapped around one of the white wires? That makes it "black". What you need to do is carefully determine which cable is hot and which cable is the switch loop to the light. Do you have any testing equipment, like a VOM?

There wasn't tape wrapped around one of the white wires, but one was wrapped together with the other 2 unused black wires.

I do have a contact line tester. Should I be getting a non contact tester for this?

Thank you!