Meaning: 'Substance' refers to a particular kind of matter with uniform properties, in this case, relating to dark matter.

Wrong options:

• weight: Weight refers to the force exerted by gravity on an object, which does not fit the context of describing dark matter.
• appearance: Appearance relates to how something looks, which is irrelevant when discussing the nature of dark matter.
• space: Space refers to the vast expanse in which all matter exists, but it does not specifically denote a type of material like dark matter.

Correct answer: observed

Explanation: The verb 'observed' is correctly used in the past participle form, indicating that scientists have not yet seen or detected dark matter directly. It fits grammatically and contextually as it relates to scientific investigation.

Meaning: 'Observed' means to watch or notice something, especially in a scientific context, indicating that dark matter has not been directly detected.

Wrong options:

• contacted: Contacted implies communication, which does not apply to the detection of dark matter.
• configured: Configured means to arrange or set up, which is not relevant to the act of observing dark matter.
• designed: Designed refers to creating or planning something, which does not fit the context of scientific observation.

Correct answer: impossible

Explanation: The word 'impossible' fits grammatically as an adjective describing the state of detecting dark matter. It aligns with the context of the sentence, which discusses the challenges of detecting something that does not interact with light.

Meaning: 'Impossible' means not able to occur, exist, or be done, indicating that detecting dark matter is beyond current capabilities.

Wrong options:

• susceptible: Susceptible means likely to be influenced or harmed by something, which contradicts the idea that dark matter is undetectable.
• resistible: Resistible means able to be resisted, which does not apply to the context of detection; dark matter cannot be resisted if it cannot be detected.
• liable: Liable means likely to experience something, which does not fit the context of discussing the detection of dark matter.

Correct answer: exists

Explanation: The verb 'exists' is appropriate here as it indicates the presence of dark matter, fitting grammatically and contextually within the sentence structure. It aligns with the scientific assertion about dark matter's gravitational effects.

Meaning: 'Exists' means to have actual being or reality, affirming that dark matter is believed to be present based on its effects.

Wrong options:

• spans: Spans refers to extending across a distance, which does not relate to the existence of dark matter.
• evaporates: Evaporates means to turn from liquid into vapor, which is irrelevant in the context of dark matter.
• depends: Depends implies reliance on something else, which does not fit the assertion about the existence of dark matter.