I can say with a very high degree of confidence that Signal does not have a backdoor. What's far more likely is that you are a victim of a misinformation campaign. In reality there are many organizations that would prefer you that you use something that isn't end to end encrypted.

That's not to say that Signal is flawless as it does have some known weaknesses. The issue is that people tend to fall back to not using encrypted messaging at all which is much worse.