What You Did
You grew a batch of bacteria. Or some different kinds of bacteria. Or fungus. Or viruses. Or a fun mix of all three!
You should have noticed the appearance of a color and texture or two inside the petri dish. These are microscopic organisms; some mixture of bacteria, viruses, and yeast (otherwise known as fungus... yes, like a mushroom!)
It is probably not possible to have zero growth after a couple days. You may have not have mixed the agar correctly.
Try swabbing as many different surfaces as you have petri dishes!Before you swab a surface, clean it with a disinfectant first (window cleaner, rubbing alcohol, bleach, etc. with parental supervision). Let the cleaner dry, then swab. You can test the effectiveness of different cleaners this way. You can also try different lengths of time between drying and swabbing.