N3
～というのは事実だ
～というのはじじつだ
It is a fact that ~, There is no doubt that
この人が私の母親を殺したというのは事実だ。
It is a fact that this person killed my mother.
Explanation
「～というのは事実だ」 means 'it is a fact that...'. In reply to doubts, rumours or a situation that cannot be avoided, you seriously admit it is true, with no excuses. In writing it often appears as 「～というのは事実だが、しかし…」: first honestly accept a fact, then give your real opinion or answer. The tone is formal, for writing, official interviews and press answers.
Mixed up with
In reading passages, when the author writes 「～というのは事実だ」, it is usually not the main point but a setup for a turn. What follows 「しかし」, 「だが」 or 「とはいえ」 is the author's real point.
It is much more formal than casual 「本当だよ」, so it does not suit relaxed chat.