Most of the universe, the Doctor says, is undetectable. Dark matter and dark energy elude every scientific instrument; they can only be inferred.
Which isn't, in Fitz's private opinion, as strange as all that. It's like trying to understand a person. The important things are hidden, and you guess them by their effects. Smiles, sulks, jokes, tirades, obvious lies and subtle ones, kisses, cups of your favorite tea or the kind he knows you hate. The gravitational interactions of Fitz Kreiner and the Doctor.
The Doctor's dark to himself, too. Unremembered centuries drift inside him, a mysterious distorting force that bursts now and then into brief illuminations. But if it were all visible, he'd probably go blind.
The thing about the universe is, it works. Stars shine, life evolves, two and two make four. Who cares if it's all dark matter and superstrings underneath?
Fitz lives well enough by guesswork.