We don't really have to guess a lot about what it was.
Matt 24:15 "Therefore when you see the ABOMINATION OF DESOLATION which was spoken of through Daniel the prophet, standing in the holy place (let the reader understand),
Jesus says this is the same type of abomination of desolation that Daniel spoke of. So what does Daniel say?
Dan 11:31 "Forces from him will arise, desecrate the sanctuary fortress, and do away with the regular sacrifice. And they will set up the abomination of desolation.
Dan 11:36 "Then the king will do as he pleases, and he will exalt and magnify himself above every god and will speak monstrous things against the God of gods; and he will prosper until the indignation is finished, for that which is decreed will be done.
Dan 11:37 "He will show no regard for the gods of his fathers or for the desire of women, nor will he show regard for any other god; for he will magnify himself above them all.
Dan 12:11 "From the time that the regular sacrifice is abolished and the abomination of desolation is set up, there will be 1,290 days.
2Thes 2:3 Let no one in any way deceive you, for it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction,
2Thes 2:4 who opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the temple of God, displaying himself as being God.
We know it had to do with the sacrifice in the temple. We know the sanctuary will be desecrated. We know the "regular sacrifice" will be done away with.
This part isn't in the Bible, but it is in some Jewish historical records. Antiochus IV Epiphanes desecrated the first temple by sacrificing pigs (swine are the most unclean animal in Judaism) This happened in 167 BC.
This is mentioned in the books of Macabees (not canonized in Protestant Bibles) and is the main thing that led the to Macabean revolt.
1 Macabees 1:54 specifically mentions the "abomination of desolation".
Some Christians think Hanukkah has to do with Christmas. But it's actually the celebration of when the Jews recaptured and rededicated the temple. (The Jews don't recognize Christmas).