The evidence includes:
1. Statements by thirty different groups of witnesses, three separate groups of police officers, documenting the sight of a large object flying at low altitude. An estimated 13,500 people on the ground saw the object, of which 2,600 filed written statements describing in detail what they saw.
2. Detection on radar of the object from two F-16 Fighter jets scrambled to intercept the object
3. Secondary detection of the object from ground radar at a nearby military airfield
4. Data from the F-16s suggesting near impossible maneuvers. Quoting wikipedia:
"During the first radar lock, the target accelerated from 240 km/h to over 1,770 km/h while changing altitude from 2,700 m to 1,500 m, then up to 3,350 m before descending to almost ground level – the first descent of more than 900 m taking less than two seconds. Similar maneuvers were observed during both subsequent radar locks. On no occasion were the F-16 pilots able to make visual contact with the targets [NB: it was around midnight hence poor visibility] and at no point, despite the speeds involved, was there any indication of a sonic boom."
5. A low quality photo, which may have been faked.
With regards to the evidence presented, I find it very difficult to come up with a convincing explanation of this incident.