Below are given some sentences out of which the sentence numbered 4 has been correctly placed. The rest of the sentences A, B, C, D, E, F need to be arranged correctly in order to form a logical order.

A. Dark matter is assumed to be non-baryonic in form, probably being composed of some as-yet-undiscovered subatomic particles. 

B. Dark matter is called dark because it does not interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum, making it almost impossible to detect using normal astronomical equipment.

C. The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do if they did not contain a large amount of unseen or dark matter.

4. One can safely assume that the primary characteristic of dark matter is some new kind of elementary particle that has not yet been discovered, and which interacts weakly with massive particles.

D. Dark matter is a form of matter that is found in galaxies and is thought to account for most of the matter in the universe and about 25% of its total energy density.

E. For this reason, most experts think dark matter to be abundant in the universe and to have had a strong influence on its structure and composition.

F. Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen.