Is the dagger that Macbeth refers to in his soliloquy visible audience?

The dagger that Macbeth refers to in his soliloquy is not visible to the audience. It is a figment of Macbeth's imagination, and is only seen by him. This is evident from the fact that he says, "Is this a dagger which I see before me?" if the dagger were actually there, he would not need to ask this question.