I'm still learning. This is Goku seeing artificial light for the first time in second chapter of DB. Translated as: "It's daylight in here!"
こん中 = この中 = in here
だけ = just
昼間に = daylight
なった = turned to
ぞ = emphasizer
I don't understand the nuance that だけ is adding to the sentence. There would be any difference in meaning if it is replaced by は?
Bulma had just turned on the lights. Is this だけ maybe referring to this fact but it got lost in the translation?.