I wouldn't go for the line some have said about how he wasn't nice so you moved away. That's still a lie and will make her feel like she's half made of a not nice person, and also jump to conclusions later like you were raped. Honestly, just say that you don't know who her father is, that sometimes that happens and she'll understand when she's older, but for now you love her etc.
I think focusing on her father not being nice is not only continuing to lie - because you don't know who he is and no doubt some would say you 'weren't nice' back then, but clearly people change, have different dimensions and you really want to stay away from anything that will make her feel more insecure and like she's less loveable.
If it's a situation where you genuinely never knew who the father was and they don't know that they're a father either, then the best line to take for now is that you don't know who he is and that's okay, she's very loved and very wanted.
I say all this as someone from a different situation but who has never known who her biological father is and never tried to find out via DNA sites etc., because my mum and her DH were my parents as far as I was concerned. Without a father figure in her life, your DD will likely be more curious to seek out that side of her family so it will probably happen one day, maybe in her teens, and you can deal with that then. But the more lies you tell now, the more they'll come back to haunt you, so don't be tempted to cover up too much.