I was thinking about the famous equation E=mc˛ the other day, when I realised I didn't actually understand it, or rather, the units used.

It seems to me that Joules = (volume multiplied by density) multiplied by (300 million meters divided by 1 second) multiplied by (300 million meters divided by 1 second)?

I know there's something I must be missing, but I can't wrap my head around how it all fits together.