Ok first, this isn't how proof works. You can't ask people to prove a negative, you, as the person who has a hypothesis, are the one who has to prove your claim.
Second: all this stuff is supposed to have happened before Jesus, so over 2000 years ago. If nephilims, people who were interbred with fallen angels did exist, we would ALL be some degee of them because if you go back far enough WE are all related. Would be able to identify some weird stuff in our DNA today and it would immediately be proven if that were the case.
Third: interspecies breeding is not possible. You can't breed a cat and a dog and get a doggat. By saying that these things could breed with humans you are essentially saying that they are also human.
Fourth: Say they did exist, and some people knew about. Why hide it? Why would anyone even want to hide it? How many people have to all behave and hide it for it to remain hidden for over 2000 years? And how come Joe shmo on the internet who doesn't know 300's about Sparta can find this out if it's been so well hidden? It can't have remained hidden for that long but also have been that easy to find.
Balance of probability says they do not exist, that they can't exist, they're just stories like vampires or werewolves. Stories like this are fun to explore in fiction, but actually believing them for your real life leaves you vulnerable to actually dangerous conspiracy theories like the ones peddled by Qanon. You are weird for believing this stuff because it's only one step removed from all the blood-libel related conspiracies that would make you dangerous to your fellow living breathing real man.