～というのは事実だ

～というのは事実だ is used to assert that a claim or proposition you or someone else has mentioned is indeed true. It adds a tone of confirmation or emphasis, differentiating what is claimed from other possibilities or rumors. It is often used when you want to firmly state that a situation or fact is the case, sometimes after reporting or quoting someone. The clause before というのは事実だ can be in plain form (short, direct) or a noun phrase; the important part is that the entire clause is presented as a fact. However, avoid overusing it with casual claims; it carries a somewhat formal, assertive nuance.