There are various quotes in scriptures which say: 'Devi is everything and Devi is all that exists'.
Some of the verses are:
In the Durga Saptashati, Markandeya Purana when Sumbha accuses Devi of killing Nisumbha by using power of others she replies as:
एकैवाहं जगत्यत्र द्वितीया का ममापरा।
पश्यैता दुष्ट मय्येव विशन्त्यो मद्विभूतयः॥५॥
I only exist in this entire Universe. There is no one besides me. Oh wicked one it is myself who is created in many forms.
In the Uma Samhita of Shiva Purana in the chapter manifestation of Devi, she says:
परं ब्रह्म परं ज्योति प्रणवद्वन्द्वरुपिणी ।
अहमेवास्मि सकलं मदन्यो नास्ति कश्चन ।।
I am the Supreme Brahman the great lusture in two Pranavas. I am everything that exist, nothing exists besides me.
विरञ्चि सृष्टिकर्ताऽहं जगत्पाताऽहमच्युत ।
रुद्रः संहारकर्ताऽहं सर्वाविश्वविमोहिनी ।।
I am the creator Brahma, protector Achyuta and destroyer Rudra and also enchanter of the whole world.
In the Saptashati, Gods pray Devi by saying that even Hara (Shiva) is unable to express miracles of Devi:
ब्रह्मा हरश्च न हि वक्तुमलं बलं च।
नाशाय चाशुभभयस्य मतिं करोतु॥४॥
Thy miracles are unparalleled, Bhagavan, Ananta, Brahma, and Hara are unable to express it. Mayest thou, O Chandika! endeavour to preserve the world, and dispel the fears of the giants.
In the Uma Samhita of Shiva Purana, in the chapter manifestation of Devi, she says that even Brahma, Vishnu and Shiva can't boast before her:
न ब्रह्मा न मुरारातिर्न पुरारातिरीश्वरः ।
मदग्रे गर्वितुं किंचित्का कथान्यसुपर्ववाम् ।।
Neither Brahma Nor Vishnu nor Shiva can boast before me, what to speak of other Gods.
In the Devi Suktam (Vak Sukta) of RigVeda Devi says that it is she who holds all this existence:
अहमेव वात इव प्रवाम्यारभमाणा भुवनानि विश्वा।
परो दिवा पर एना पृथिव्यैतावती महिना संबभूव॥८॥ (RigVeda 10.125)
I breathe a strong breath like the wind and tempest, the while I hold together all existence. Beyond this wide earth and beyond the heavens I have become so mighty in my grandeur.
In the Lalita Sahasranama also we have:
सृष्टिकर्त्री ब्रह्मरूपा गोप्त्री गोविन्दरूपिणी ॥ ६३॥
संहारिणी रुद्ररूपा तिरोधान-करीश्वरी ।
सदाशिवाऽनुग्रहदा पञ्चकृत्य-परायणा ॥ ६४॥
who creates world by taking form of Brahma, who preserves by taking form of Govinda, who destroys by taking form of Rudra, who veils (Tirodhana) by taking form of Ishwara and who graces through SadaShiva, who does all theses five functions (Panchakritya).
You can read various stories of Devi in Devi Bhagavatam, and Devi Gita also if you want to read more. Similarly contemplating on the meanings of Lalita Sahasranama also reveals many things!