To accurately determine the author's overall purpose in the text, I would need more context or specific excerpts from the text in question. However, based on the options provided:
- If the text delves into the current understanding and challenges of dark matter without proposing new theories or materials, then A or D might be the purpose.
- If it focuses on the composition of dark matter, then B is likely the correct choice.
- If there is a presentation of new hypotheses regarding dark matter, then C might be the goal.
Please provide more details from the text for a more precise answer.