Nothing -
What we can't touch

One of our habits is to believe something only if we touched it ourselves. But sometimes this could be difficult, because not everything that is visible can or should be touched. If it would be hard for us then to grasp these things?
 

    because it will burst

Of course we are able to see it, as it shines with all colours of the rainbow, and, supported by air, performs a little dance. Nobody will doubt about its existence, because we created it maybe ourselves, but as soon as we want to touch it, it won't be there anymore, the soap bubble.
 

    because it will melt

We can touch snow - given we are not too sensitive to coldness. But that won't go well for too long. Either it will become too cold for us, or, in case we are of the tough kind, the snow will clear off - in liquid form. And if the snow won't melt, then this is one of the rare cases when we are close to freeze to death.
 

    because it may be hungry

As hungry as a tiger, for example (that only gets fed with cereals).

Or a Tyrannosaurus Rex. As emaciated as it seems, it would be able to swallow an entire herd of cows in just one leap.

Or a pond, seething with a pack of greedy monster prime carps. If you there put your finger in the water, you must be a death defying daredevil. Or at least a little thoughtless.
 

    because it is too far away

Even if we could manage to get close enough, it may be quite difficult to touch condensed atmospheric humidity. Unless the water is frozen. But then it would, as we already learned, melt again.
 

    because it is not really there

Just like this moment. We can see it, but it is already long gone. And even then we wouldn't have been able to touch it. Quite strange that it is nevertheless possible to take a picture of it.
 

    because we don't exactly know what it is

And as long as we don't know, what it is, we better don't touch it. The vicious circle of ignorance.
On the first picture, I don't know it exactly, but I think these are juice-extracted chicken. On the second picture, I know exactly that it is very fluffy and keeps warm.