well I think that answer is atoms but energy could also be a factor then again pure energy is massless so no matter.
Step-by-step explanation:
as we all know atoms are indivisible units of matter they are the smallest unit of anything well that and quarks but yeah. gravity can be defined as one-dimensional entity as it spread throughout the universe similar to that is Force but as you know exist in many forms and has directions but Force in itself does not contain matter but uses forms of matter and any other forms of energy to generate .