Daniel is not all by himself for other people were with him. The others do not see the man in linen. Yet something so impressive happens that those others tremble greatly and hide themselves. It makes you think of the appearance of the Lord Jesus to Paul in Acts 9. When the Lord Jesus appears to Paul there in his divine glory and speaks to him, we read in Acts 9:7, The men who were traveling with him stood speechless, hearing the voice but seeing no one.
7 And I, Daniel, alone saw the vision, for the men who were with me did not see the vision, but a great trembling fell upon them, and they fled to hide themselves.