The phrase "a substantial unknown" is correct and usable in written English.
It can be used when referring to a significant amount of information or factors that are not known or understood in a particular context.
Example: "In the realm of scientific research, there remains a substantial unknown regarding the long-term effects of the new drug."
Alternatives: "a significant uncertainty" or "a considerable mystery".
A substantial population of unknown, small-sized objects was found in these surveys.
Crude oil is a complex mixture of a range of different components like aliphatic and aromatic hydrocarbons (PAHs), phenols, and a substantial amount of unknown compounds [ 1].
Though our results focused on a previously characterized CNV, they raise the possibility that there could be a substantial problem across unknown regions of the genome with common CNVs.
In particular, we recovered a substantial number of unknown genes encoding putative secreted and transmembrane proteins, suggesting new components of signaling pathways that might be incorporated within the existing regulatory networks controlling D. melanogaster embryogenesis.
Without having access to statistical repeat prediction by MDR analysis we previously assumed that the sequence contained a substantial proportion of yet unknown TE sequences.
In addition to the reannotation of known genes, a substantial number (10,884) of unknown transcripts (UTs) were discovered, which had not previously been annotated.
Although we have uncovered a substantial number of previously unknown hybrids, previous authors have come to similar conclusions about some of the few specimens known previously (Additional File 2).
William Friedkin's disturbing thriller was curiously overlooked upon release in 2006 despite a career-best performance from Ashley Judd and a substantial role for a relatively unknown Michael Shannon.
In-situ micro-Raman study revealed that Joule heating near the electrical breakdown gave rise to a substantial structural evolution: a previously unknown broad amorphous-like and partially reversible phase at an on- and off-current of ∼3.0 × 108 A/cm2, which finally drove the phase to the electrical current-breakdown.
However, there was only a short period of demolition data and a substantial amount of properties with unknown ages.
