Question: I've been having this doubt for a quite a while and haven't been able to find a satisfying answer. Is the 'E#' note technically an F? If so, why does it not exist on the chromatic scale like other sharps? I mean, what exactly is the E#? Is it a note on its own or is it just another way of saying F?

And what do people mean when they say that the D# Major scale is theoretical major scale and why it does not exist in the circle of fifths diagram?

I came across the last paragraph on a website.

Answer: 
1. E-sharp is called the enharmonic equivalent of F-natural. The sharp raises the base note one half step. In the same way B-sharp is the enharmonic respelling of C-natural. Furthermore, C-flat is the enharmonic of B-natural, and F-flat enharmonic of E-natural. Notes may also be double-sharped and double-flatted, which raises the base note by two half steps.
2. The key of D-sharp could be considered a theoretical scale because it doesn’t appear on the basic circle of fifths is simply because it is a scale that contains these enharmonics, and traditionally composers write this as E-flat to avoid the enharmonic spelling of the second scale degree, F-natural, which is enharmonic with E-sharp, the second degree of D-sharp major, and no one likes reading parts with double-sharps in the key signature (shudder). The seventh degree of the D-sharp major scale is C-double-sharp. D-sharp major chords definitely exist in music and we play enharmonic spellings all the time.
For starters, there isn't an E# in the D major scale. There's E and there's F#. If you're asking why there may be E# found in a piece that's in D major, that's slightly different.

Sometimes certain notes that are non-diatonic need to be used. That's maybe where E# could come in. Let's say the chord sequence goes from D major into F# minor. There's often the V of the new chord that precedes it. Here, it would be C# or C#7. Now the spelling for that is C# E# and G#. It is **not** spelled C# F and G#.

There are other times and reasons why some notes are written in 'strange' ways, but there are generally good technical reasons for them. And just because these notes happen to be white keys on a piano doesn't mean they can't be called # or b. These being - B#, Cb, E# and Fb. Of course, for beginners, things like E# sound pretty silly, why not F instead? Correctness and technical reasons!

**EDIT** - now the question has been edited, this answers a rather different one! Thanks! I'll leave this up for now, (OP originally asked about E# in a D major key), as it goes a little way. Luckily, I noticed the edit, otherwise I'd look even more stupid than usual.