。
Nobody had ever heard about it.
Literal
That [about-について] heard-experience-exists person [topic-は] nobody was-not.
~たことがある expresses life experience — 'have ever done X.' Here in the negative (いなかった), meaning nobody had the experience of hearing about it. 誰も…いなかった is the standard 'nobody' pattern: 誰も (anyone) + negative verb.