Good day.

In addition to GATC , is there other 'filler' compounds lodged between them, and other variations different than sugar phosphate constituting the 'back bone' shown here, or eventual contaminants to the purity of any of these components ?

Generally speaking no- DNA is a polymer consisting of nucleotide subunits. Each nucleotide consists of deoxyribose (the pentose sugar), a phosphate group and a base.

That being said, some parts of the DNA can be modified by adding e.g. a methyl group. In vivo there is also water, but that is not really part of the DNA structure. If you are thinking of "filler" because the image makes it look that there is a lot of space there, this is in large part due to the schematic nature of the image. If you do an xray of a DNA crystal, you do not have a lot of space, the base stacks are sitting quite neatly on top of each other.

Interestingly I have done many simulations of DNA and RNA and thought the question warrants more than the generic answer. In general, like CharonY points out, the actual space filled by all the atoms leaves very little room, however there are ions that fill in the grooves some times transiently, mostly K, Ca2+, Na and Mg2+ is like a magnet. High concentrations of Mg2+, non-natural actually cause DNA cleavage with nothing other than water. Overall the structure is very fluid, think of it as having small ripples that move down either strand constantly. Aside from this, there are several natural nucleotide analogous, uricil for instance in RNA can also be incorporated but rarely, usually only from a diseased state. But there are a wide range of synthetic DNA like molecules, where the sugars or backbone phosphates have been replaced, such as PNA (protein nucleic acids), or modified sugars, but these are all synthetic as mentioned. There are some pathogens that work by modifying DNA into Urcil, or derivative nucleic acids through cytosine mostly, or thymadine. There are also many compounds that work by intercalation between nucleotides, most cause cancer. An example most used is ethidium bromide, used in labs to see DNA in agar gels, where the molecule exploits the kinetic movements I mentioned, and works it's way in between base pair stacks.

I think for those things one would need to start looking at the structure in more detail. DNA is fairly flexible but much is due to the helical backbone. Intercalation between base stack does bend the structure for example, but imagine it as "filling" is, at least to me, more confusing. It is more inserting into tight stacks and creating a partial unwinding of the helix or sometimes a just a bending.

Also, not terribly important, but since students sometimes do that, I have to note that one shouldn't confuse agar with agarose. Especially if one does not want to spend time cleaning up the gel chambers.