The mass-energy content of the Universe is approximately this: 5% normal matter, 20% dark matter, and 75% dark energy. Because we don't know much about dark matter and dark energy, apart from the fact that they should exist in order to explain observations, it is often stated that we only know 5% of our Universe. But isn't this a logical fallacy? This is only the mass-energy content of our universe (as obtained with our current understanding), it is not a degree of how well we understand our universe.
This argument hinges upon the notion of what it means 'to know', and it uses know in two different senses; this makes it a fallacy of equivocation.
Merriam-Webster lists a number of definitions of the verb 'to know'. When one begins arguing about knowledge and conflating the different sorts, one equivocates. This is a subtle informal logic fallacy because it's natural to accept both definitions as acceptable and meaningful making it persuasive. In your example:
- To know the composition of the universe would appear to be 1a(3) "to recognize the nature of; discern".
- To know rules which govern our known universe would appear to be 2b(2,3) "to be acquainted or familiar with", "to have experience of".
So let's simply reformulate your example argument:
P1. We have measured the (physical) stuff of the universe and and according to mathematical models, only 5% is normal matter we can interact with directly with our senses.
P2. We can only understand, be familiar, and have experience of normal matter.
C. Therefore, we can only know 5% of the universe.
There are many problems with this rewritten argument. Does measurement of composition based on mathematical abstraction actually characterize recognizing the nature of something? Must we interact with something to understand it? Perhaps dark matter and energy are a near uniform field and obey deterministic laws yet to be discovered. What does it mean to understand, be familiar, and experience any matter? Even if we can only understand, be familiar, and experience normal matter, the universe is a large place, so how can we claim to have a full knowledge of that 5%? How is using a machine to understand leptons any different than using one to understand dark matter or energy? Do we actually experience leptons?
We could go on, but all of these hinge upon what it means 'to know'. In philosophy, there are two important disciplines of distinctions about knowledge and things called epistemology and ontology, respectively where for thousands of years thinkers have toyed with different ideas of 'to know' and 'to exist'; the argument you have presented is so coarse as to almost be meaningless.
This seems like a semantic issue to me.
Under an interpretation of "We only understand 5% of our Universe." as "There is only 5% of the universe [the regular matter] of which we have [significant] understanding." seems perfectly reasonable as a qualitative statement about our state of knowledge, and thus not a logical fallacy.
The "5%" is a characterization of the "portion of the universe" of which we have some understanding -- not a description of the amount or quality of the understanding itself.