Brief Answer:
Floaters.

Detailed Answer:
Hi
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These can be because of floaters. Floaters are dark specks in the form of dots, circles, lines, flashes or cobwebs that seem to move across your field of vision. Floaters are small clumps of visible material within the vitreous. Usually they remain clumped but in old age bits of debris are discarded by the vitreous humour or retina that causes protein (connective tissue) to be released.Hence crystal-like deposits may form in the vitreous and interfere with light passing from the front of the eye to the back.These floating particle within the vitreous are seen as shadows in retina.The vitreous sometimes pulls on the retina. This may cause you to see flashing lights or lightning streaks, even though there aren't any such lights present. When the vitreous separates from the retina.
Detailed retinal examination plus slit lamp examination of eye should be done by opthalmologist.
Thanks.

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