Grunfeld defense

The grunfeld defense runs:1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.cxd5 Nxd5. But now if 5.Nxd5 Qxd5, what can go wrong for white when he plays 5.Nxd5?

I know you'll tell me that the queen's knight no more exists to push the black queen at d5.But I want to ignore theory and know the moves Black must make to gain the advantage because, you know, when you play chess only according to theory,when the opponent makes an unusual move, you can sometimes find yourself in a  difficult situation.

So what can black do  here after  6.Nf3?

What can go wrong for white?  The b2 pawn has not moved to c3 where it bolsters d4.  Pressure on d4 is thematic in the Grunfeld. 

What can black do after 6 Nf3?  Black has no worries.  His queen occupies a strong position and he can pressure d4.

After only 5 moves black has already an equal position , so something has already gone wrong for white. Maybe you mean what else can go wrong ? 

Yes as ghostofmaroczy said, there is no white pawn on c3. and in few moves (maybe after Bg7 & o-o) black is going to push c7-c5, since white cannot further defend the d4 pawn, he will have to exchange on c5. which leads to a totally equal position. especially with the same pawn islands on both sides.

Ok I think I've understood.I had not thought about the c5 pawn.But there's another thing that annoys me in the grunfeld.Sometimes Black keeps his dark squared bishop on the diagonal a3-f8 instead of fianchettoing it even though the g6 pawn has been pushed.How can this be better than fianchettoing the bishop?I cant remmember the position but I think you know what i'm talking about.

Kasparov and Karpov liked this opening when playing in the 1987's

Seville tournament in Spain?  Why?  Equal chances for white and black?

Maybe I guess...

HypnoDisk,

That was the greatest match of all time.

The Seville Variation of the Grunfeld that Karpov and Kasparov contested is a fascinating variation.

Equal chances?

Not in the slightest.

Advantage to the better chess player all the way.