Data demand is unknown. vs The data demand is unknown.
Data generated by existing methods is ... vs The data generated by the existing methods is ...
2 Answers
1) Data demand is unknown.
Here demand is uncountable.
- uncountable noun
If you refer to demand, or to the demand for something, you are referring to how many people want to have it, do it, or buy it.
- could be either one. It depends on whether you are referring to a particular data set, or data in general.
Data demand is unknown. vs The data demand is unknown.
You can use either, 'The' can be omitted as demand is uncountable. However, the second has more stress on the specific 'data demand', especially if you have mentioned the demand in your previous content.
Data generated by existing methods is ... vs The data generated by the existing methods is ...
Similar, if you have mentioned the existing methods before, you can use 'the' to emphasis them. The first 'the' can also be omitted in this case as data is a plural noun. Using 'the' gives more stress on 'all the data' generated by the methods. Speaking of 'data', you should use 'are' instead of 'is', otherwise you should use datum.