Today apps getting more and more greedier for memory. Many already know that Dark series motherboards are 1 DPC (DIMM Per Channel) boards to help with best memory performance possible at cost of max capacity. This was a deliberate choice as adding additional slot make memory traces longer, and leaving unconnected empty slot cause reflections and bad signal quality for CPU's memory controller. But time move on and today we can buy 32GB UDIMM non-ECC. Latest BIOSes already implement update to support these memories in current DARK motherboards. 
 
Keep in mind that cheaper 32GB+ RDIMM (Registered) or ECC DIMMs will NOT work , because these need server Xeon CPUs and specific layout, which is absent on DARK boards, again to improve overclocking margins.
 
Supported 32GB memory list:
 
EVGA X299 DARK (4 DIMMs, 1 DPC, maximum memory capacity 128GB with 4x32GB UDIMM) starting from BIOS 1.19
* Samsung 2666MHz UDIMM M378A4G43MB1-CTD - I bought mine on eBay for ~$160 a pair
* Corsair Vengeance LPX 32GB module kits - available from $150 per module, depends on speed grade
32GB support is not relevant to specific CPU, these RAMs can work equally well with oldest Skylake-X like 7980XE, or 9980XE or new CascadeLake-X. 
 
EVGA Z390 DARK (2 DIMMs, 1 DPC, max capacity 64GB)
* Samsung 2666MHz UDIMM M378A4G43MB1-CTD
* Corsair Vengeance LPX 32GB module kits
32GB support is not relevant to specific CPU, these RAMs can also work with older CPUs like 8700K or newer 9900K/9900KS, etc. 
 
EVGA SR-3 DARK (6 DIMMs, 1 DPC, max capacity 192GB with UDIMM).
This board is bit special, because it's the only one that can work with BOTH UDIMMs or RDIMMs but not at the same time(mixed). 
More about this later.
* Samsung 2666MHz UDIMM M378A4G43MB1-CTD
* Corsair Vengeance LPX 32GB module kits
 
If somebody successfully tested and overclocked other 32GB modules on these or other EVGA MBs  - feel free to comment, I will add your results into this post.
 
Despite low spec frequency 2666MHz these Samsung modules can overclock really well, given the capacity. I was able to get some 3Dmark runs with 3700MHz, with reasonable timing/voltage bump. With more finetuning and better IMC perhaps stable clocks around 3800-3900MHz are possible.
 



 
BIOS screenshots with 7980XE on X299 DARK (I had only 2 DIMMs to test):
 





 

No, G.SKILL DC modules are essentially two DIMMs on one PCB, requiring special receipe to work.
They are not supported on EVGA boards. Such zombies also not good for overclocking, as they present more than double of the loading on the IMC.

According to this News "ASRock X299 Motherboards To Feature Up To 2 TB RDIMM DDR4 Memory Support" is it possible to use 4x64 or 4x128Gb UDIMM memory with x299 Dark? Something like this.

I cant imagine what youd want all that ram for other than a ram disk maybe. What would be really beneficial is if we could get some apps/games that use more than 4 cores and do it proficiently, not just paintshop. Were all dressed up with cores and no where to go with them!

You will be fine using faster rated modules at slower speeds.