In general, there are two kinds of spiritual energies: holiness, or Kedushah, and unholiness, or Kelipah (sometimes referred to as the “Samech-mem”). These are inherently spiritual realities that exist beyond and independently of the physical world. As such, they are not directly perceivable by regular mortals, whose domain is the physical world.
Everything physical depends upon a certain spiritual energy for it to exist as it does. It is thus taught, “There is no blade of grass down below that does not have an angel on high that strikes it and tells it to grow.” So there are two kinds of physical beings in the world: Those that receive their spiritual sustenance from holiness, or Kedushah, and those that receive it from unholiness, or Kelipah. There is no middle ground, and so any physical being that is not holy necessarily receives its sustenance from Kelipah.
To restate it, Kedushah and Kelipah involve two parts, respectively: there is the spiritual energy, and then the physical entities in the world that draw their sustenance from that energy.
The forces of Kelipah possess a tremendous amount of energy, energy that sustains all the beings in the world that draw their sustenance from it, from all the four kingdoms of domem (inorganic things), tzomei’ach (plant life), chai (the animal kingdom), and medabeir (mankind). However, their energy is not their own—it is derived exclusively from holiness. Hence, the existence of Kelipah and those physical things that receive their sustenance from it, is not a true existence, and so they are compared to darkness, which has no substance.
This is also why the Kelipah is compared to the gnat, whose entire existence is derived from sucking life from other creatures, and which lacks any capacity of its own to give. Similarly, the Kelipah is compared to the leech, whose life depends upon sucking blood from its host.
Likewise, the Kelipah receives a certain minimum amount of sustenance from Kedushah that enables it to maintain its existence. However, the Kelipah is not satisfied with this; it greedily yearns to boost its strength as much as it can. Since it lacks any energy of its own, its only way of growing is by drawing extra energy from Kedushah, which it accomplishes by preying on those who draw their sustenance from Kedushah.
The Kelipah sucks this extra energy from Kedushah by enticing the Jew to violate one of the 365 negative commandments.
 Bereshis Rabbah 10:6.
 Tanya ch. 6.
 Cf. ibid. ch. 24, beg.
 Ibid. ch. 29.
 Ibid. ch. 24.
 Toldos Avraham Chaim, ch. 9.
 Tanya chs. 4, 37.