File system encryption with plausible deniability

Question

When we use file system encryption, even though attackers won't be able to access the information we've stored, they would be able to find out the fact that we have encrypted data on our drive. So the question is, are there ways to hide even the existence of encryption to an attacker with full access to the storage device and reasonable resources, so that to their eyes, the encrypted data are just unused space.

It would be best if the following can be answered:

1. Are there research on this subject? What's the scholarly term?
2. Are there theoretical limitations on this? Is such encryption possible?
3. Are there any known implementations?

PS: What I'm asking is stronger than deniable filesystems: the existence of encryption of any sort must be plausibly hidden, so that reasonable attackers won't even attempt decryption, not just the existence of specific content.

Answer 1

With VeraCrypt's Hidden Volumes there is plausible deniability of your actually protected data. But you can apparently only have a hidden volume within another veracrypt volume. So the attacker will know that you are using encryption, but he will not know if you have a hidden volume. You could maybe even maintain several hidden volumes, so you could pretent to given them the password to your hidden volume but it will not be the correct one.

Otherwise, you could have one unencrypted partition on your hard drive which you boot from normally. And then have some other seemingly unused partitions which are in fact encrypted VeraCrypt volumes. But I guess such unused partitions may already be suspicious to the attacker.

Or have your encrypted volume on an external drive which you pretend to have been recently erased. I am not sure, though, if the header data of encrypted drives will give aways something about the fact that they are encrypted.

UPDATE: If you are looking for methods to hide information (encrypted or not) within non-encrypted data, maybe digital watermarking has some interesting approaches.