There are no original manuscripts for any part of the Bible. However there is strong evidence that the prophecies in Daniel were written at about that time.
To summarise some of the evidence:
1) The book is classified among the "Writings" rather than the "Prophets". If Daniel had been written when Daniel supposedly lived - even the parts dealing with Daniel's life this would be very surprising. On the other hand if Daniel were late it would be expected.
2) There is no clear reference to the book or Daniel the person prior to that date.
3) There are historical errors in the parts dealing with Daniel's life (for instance the Persians overcame the Medes prior to conquering Babylon - Daniel reverses these events).
4) The prophecies are clearly directed against Antiochus. I suggest comparing the prophecies to the other accounts of the period such as Josephus and 1 and 2 Maccabees, as well as modern historians.
5) The prophecies are wrong concerning the death of Antiochus. This is exactly what we would expect if the prophecies were written shortly before that time.