When/If he teleports to a space he cannot see, that is essentially what Kurt is doing.

Even if we invoke parallel dimensions to explain how Nightcrawler's teleportation works, however, it should not be seen as him "moving" in a physical sense. He is not "stepping thru a door" or sliding down a chute. As Matt notes, Kurt is HERE, and then he's THERE. The parallel dimension has "folded" space -- otherwise, his teleportation would not be instantaneous, as it has been so often shown to be. If he teleported a mile thru that parallel dimension, and actual movement was involved, it would take him as long to travel that mile as it would in real space.

And if that is "instantaneous", then we have changed Kurt's power from true teleportation to something more akin to super speed.

(How unhip is my spellchecker? It does not recognize "teleportation"!)

I figured Nightcrawler was a man-sized example of the tunnel-diode effect.  As Matt says, he's here, now he's there, and he doesn't exist in the space in between.  Then it got explained.  First he was "riding on magnetic lines of force" or somesuch, which allowed Magneto to know ahead of time exactly where he would appear, which means Magneto can read lines of force and respond to changes in them at the speed of light.  Phooey. 

Then he was unconsciously traveling through another dimension.  Double phooey. 

Writers often use the "unable to transport into solid objects" line when warning of the dangers of teleportation.  Star Trek often did, and Nightcrawler did as well.  Liquids and solids each have molecules that are in direct contact with each other.  Gases are mostly empty space.  Solids and liquids contain no empty space at all between the outer electron shells of neighboring atoms.  Teleporting into water should be no different than teleporting into concrete.  The bit about "lower density" makes no sense unless every atom of both Nightcrawler and the substance into which he's teleporting "know" the overall density of the Nightcrawler-shaped volume they share with other, dissimilar atoms.  Do they take a vote? 

I liked the way TOS handled teleportation.  They could do it because it worked.  They never bothered to explain it to each other, because they all knew that it worked.  We knew it worked because we watched it work.  Now on with the story.  TNG decided to tell stories ABOUT teleporting.  I think that was a mistake.

••

This is very much at odds with what I have read. Everywhere that I have seen any sort of description, the space between atoms has been given as greater, proportionately, than the space between galaxies. And the atoms themselves are mostly (very much mostly!) empty space. (Consider the old analogy that in an atom enlarged to the size of a football stadium, the nucleus would be the size of a common housefly!)

Kitty Pryde's power works (when properly portrayed, as opposed to, say, how it is shown in the movies) by diminishing the attraction between atoms such that hers can pass freely between those of whatever she moves thru -- the distances between the atoms being so great there is no danger of one atom bumping into another.

It would not be IMPOSSIBLE for one object to be materialized inside another -- there is plenty of room for all those atoms! -- but it would almost certainly be fatal. And the discrete electric fields might not get along all that well, either!

It's my understanding that solids have their constituent atoms locked in place and that liquids have atoms that "slide" over each other, which also indicates physical contact.  Whatever "contact" means on that scale.  Atoms of gases bounce off each other randomly, because they are separated by some relatively great amount of empty space.

From this link, the first paragraph defines atomic radii: We can estimate the size of an atom, however, by assuming that the radius of an atom is half the distance between adjacent atoms in a solid.

That reads to me that the atoms are touching each other.  The force fields generated by the outermost electron shells of neighboring atoms may keep them from  direct contact, but those fields "fill in the holes" so to speak, and would prevent matter (or other force fields) from appearing within them.  Kitty negates the force fields and slides "between them", but that's no different than Kirk beaming through a ceiling to appear inside your living room.  What would happen to Kitty if she turned her power off while submerged?

That reads to me that the atoms are touching each other.

••

Yikes! No!!

The radius of a circle is half its diameter. If the radius of an atom is half the distance between adjacent atoms, then that distance would be equal to the diameter of the atoms.

Considerably less than the proportionate distance between galaxies, but still far from "touching".

Atoms DO "touch" in some solids -- neutronium for instance. But that's not normal matter, where the different degrees of "solidity" are the result of different strengths in the attraction between the atoms.

••

In principle, that is what she did to the Juggernaut in X-3. This resulted in him becoming trapped in the floor into which she had pulled him down -- tho only temporarily, since in the movies her power is portrayed as if the atoms flow around her. So Juggernaut simply flexed and burst free, as Superman would if buried in concrete.

Properly portrayed, however, Juggernaut should have become "part" of the floor, his atoms intermingled with those of the cement. Thus he would have been permanently trapped -- and most likely dead.