I think The-Wolf is pretty close to as expert as you can get on danios around here. But I would check again. Yes, it is possible that you got all girls, but unlikey:
(0.5)^6 = 0.015625 = 1.5% chance you have all girls. Not likely, but certainly possible.
The PH of the tank they were spawned in affects the sex ratio of fry; mabye the tank your danios were bred in had a very high or low PH. It's also possible that your danios aren't mature yet. Usually the only way you can sex most fish is by fatness - female egg scattering fish are invariably fatter than males.