Don't think any concrete evidence exists, but the fact that the politicians (across the aisle) that were briefed on the hackings are universally accepting that

Don't think any concrete evidence exists, but the fact that the politicians (across the aisle) that were briefed on the hackings are universally accepting that Russia hacked the DNC is pretty good indication that the claims are true. Well, either that or there's a huge conspiracy going on, or gross incompetence.